2009. "Defining Active Catalyst Structure and Reaction Pathways from ab Initio Molecular Dynamics and Operando XAFS: Dehydrogenation of Dimethylaminoborane by Rhodium Clusters ." Journal of the American Chemical Society 131(30):10516-10524. Abstract We present the results of a detailed operando XAFS and density functional theory (DFT) based ab initio molecular dynamics (AIMD) investigation of the proposed mechanism of dehydrogenation of dimethylaminoborane (DMAB) by a homogeneous Rh4 cluster catalyst. Our AIMD simulations reveal that the previously proposed Rh structures are highly fluxional exhibiting both metal cluster and ligand isomerizations and dissociaton which can only be accounted for by a examining finite temperature ensemble as generated by AIMD. It is found that a highly fluxional species Rh4((H2BNMe2)82+ is fully compatible with operando XAFS measurements which suggest that this species may be the catalyst resting state. Based on this assignment we propose a catalytic mechanism for DMAB dehydrogenation which exhibits a maximum energy barrier of 24 kcal/mol, which is half that observed for the uncatalyzed thermal reaction. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program, and was performed in part using the Molecular Science Computing Facility (MSCF) in the William R. Wiley Environmental Molecular Sciences Laboratory, a DOE national scientific user facility located at the Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the U.S. Department of Energy.
2009. "Self-consistent polarization density functional theory: Application to Argon." Journal of Physical Chemistry A 113(10):2075-2085. doi:10.1021/jp808767y Abstract We present a comprehensive set of results for argon, a case study in weak interactions, using the selfconsistent polarization density functional theory (SCP-DFT). With minimal parameterization, SCPDFT is found is give excellent results for the dimer interaction energy, the second virial coefficient, the liquid structure, and the lattice constant and cohesion energy of the face-centered cubic (fcc) crystal compared to both accurate theoretical and experimental benchmarks. Thus, SCP-DFT holds promise as a fast, efficient, and accurate method for performing ab initio dynamics that include additional polarization and dispersion interactions for large, complex systems involving solvation and bond breaking. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.
2009. "Thermodynamics and Kinetics of Nanoclusters Controlling Gas-to-Particle Nucleation." Journal of Physical Chemistry C 113(24):10354-10370. Abstract Nucleation of new particles from vapor-phase molecular precursors is an important process in the synthesis of nanomaterials and in the formation of aerosols in the atmosphere. Vapor-to-particle nucleation is a macroscopic process controlled by nanoscale particles (e.g., molecular clusters). Computational approaches to nucleation have been limited by the lack of a consistent theory of the process and by the lack of efficient approaches to simulate the properties of clusters relevant to nucleation. In this article, we focus on two advances that allow nucleation to be treated in a rigorous manner for molecular systems: Dynamical Nucleation Theory permits a consistent treatment of the nucleation kinetics and aggregation-volume-bias Monte Carlo simulations using self-adaptive umbrella sampling combined with histogram reweighting provides an efficient approach to evaluate the thermodynamics of molecular clusters important in nucleation. The combination of these two approaches positions molecular computational approaches to make significant advances in our understanding of the mechanisms of nucleation, particularly in multiple component systems that play crucial roles in nanoscience applications and in the atmosphere. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.
2009. "Experimental and Computational Studies on Collective Hydrogen Dynamics in Ammonia Borane: Incoherent Inelastic Neutron Scattering." Journal of Chemical Physics 130(2):article no. 024507. doi:10.1063/1.3042270 Abstract Incoherent inelastic neutron scattering can be used as a sensitive probe of the vibrational dynamics in chemical hydrogen storage materials. Thermal neutron energy loss measurements at 10K are presented and compared to the vibrational power spectrum calculated using ab initio molecular dynamics of pure and deuterated ammonia borane (NH3BH3, NH3BD3, and ND3BH3). A harmonic vibrational analysis on NH3BH3 clusters was also explored to check for consistency with experiment and the power spectrum. The measured neutron spectra and computed ab initio power spectrum compare extremely well (50 to 500 cm-1) and some assignment of modes to simple motion is possible, however, it is found that the lowest modes (below 250 cm-1) are dominated by collective motion. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.
2009. "Neutron Powder Diffraction and Molecular Simulation Study of the Structural Evolution of Ammonia Borane from 15 to 340 K." Journal of Physical Chemistry A 113(9):5723-5735. doi:10.1021/jp900839c Abstract The structural behavior of perdeuterated, 11B-enriched ammonia borane, ND311BD3, was investigated by neutron powder diffraction measurements collected over the temperature range from 15 to 340 K and by molecular dynamics simulation. In the low temperature orthorhombic phase, the progressive displacement of the borane group under the amine group was observed leading to the rotation of the B-N bond parallel to the c-axis. The structural phase transition at 225 K is marked by dramatic change in the dynamics of both the amine and borane group that is problematic to extract from the metrics provided by Rietveld analysis of the NPD data alone but is evident in the molecular dynamics simulation and other spectroscopic evidence. This study highlights the valued added by complimentary experimental approaches and coupled computational studies.
2009. "The Oil-Water Interface: Mapping the Solvation Potential." Journal of the American Chemical Society 131(3):1037-1042. doi:10.1021/ja805962x Abstract Ions moving across the oil water interface are strongly impacted by the continuous changes in solvation. The solvation potential for Cs+ is directly measured as they approach the oil-water interface (“oil” = 3-methylpentane), from 0.4 to 4 nm away. The oil-water interfaces are created at 40K using molecular beam epitaxy and a softlanding ion beam, with pre-placed ions. The solvation potential slope was determined at each distance by balancing it against an increasing electrostatic potential made by increasing the number of imbedded ions at that distance, and monitoring the resulting ion motion. The potential approaches the Born model for greater than z>0.4nm, and shows the predicted reduction of the polarizability at z<0.4nm.
2008. "Pyroelectricity of Water Ice." Journal of Physical Chemistry B 112(20):6379-6389. doi:10.1021/jp073870c Abstract Water ice usually is though to have zero pyroelectricity by symmetry. But biasing it with ions breaks the symmetry because of the induced partially dipole-aligned. This unmasks a large pyroelectricity. Ions were soft-landed upon 1 micron films of water ice at ≥ 160K. When cooled below 140 to 150 K this locks-in the dipole-alignment. Workfunction measurements of these films then show high and reversible pyroelectric activity from 30 to 150K. For an initial ~10V induced by the deposited ions, the at 160K, the observed bias below 150K varies approximately as 10V*(T/150K)2 This implies water has pyroelectric coefficients as large as that of many commercial pyroelectrics such as lead zirconate titanate (“PZT”). The pyroelectricity of water ice, not previously reported, is in reasonable agreement with that predicted via a monte carlo simulation of TIP4P ice. This is observed in crystalline and compact amorphous ice, deuterated or not. This implies that for water ice between 0 and 150K (such as astrophysical ices), temperature changes can induce strong electric fields (~10,000,000 V/m) that can influence their chemistry and trajectories or binding.
2008. "Materials for Hydrogen Storage: Structure and Dynamics of Borane Ammonia Complex." Dalton Transactions (33):4514-4522. doi:10.1039/b718138h Abstract The activation energies for rotations in low temperature orthorhombic ammonia borane were analyzed and characterized in terms of electronic structure theory. The perdeuterated, 11B- enriched ammonia borane 11BD3ND3 sample was synthesized and the structure was refined from neutron powder diffraction data at 175 K. This temperature has been chosen as median of the range of previously reported NMR measurements of these rotations. A representative molecular cluster model was assembled from the refined geometry and the activation energies were calculated and characterized by analysis of the environmental factors that control the rotational dynamics. The barrier for independent NH3 rotation, Ea = 12.7 kJ/mol, largely depends on the molecular conformational torsion in the solid state geometry. The barrier for independent BH3 rotation, Ea = 38.3 kJ/mol, results from the summation of the effect of molecular torsion and large repulsive intermolecular hydrogen-hydrogen interactions. However, a barrier of Ea = 31.1 kJ/mol was calculated for rotation with preserved molecular conformation. Analysis of the barrier heights and the corresponding rotational pathways shows that rotation of the BH3 group involves strongly correlated rotation of the NH3 end of the molecule. This observation suggests that the barrier from previously reported measurement of BH3 rotation, corresponds to H3B—NH3 correlated rotation. Support for this work by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences Division is gratefully acknowledged. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2008. "Equatorial and Apical Solvent Shells of the UO₂²⁺ Ion." Journal of Chemical Physics 128(12):124507. doi:10.1063/1.2884861 Abstract First principles molecular dynamics simulations of the hydration shells surrounding UO₂²⁺ ions are reported for temperatures near 300 K. Most of the simulations were done with 64 solvating water molecules (22 ps). Simulations with 122 water molecules (9 ps) were also carried out. The hydration structure predicted from the simulations was found to agree very well known results from X-ray data. The average U=O bond length was found to be 1.77Å . The first hydration shell contained five trigonally coordinated water molecules that were equatorially oriented about the O-U-O axis with the hydrogen atoms oriented away from the uranium atom. The five waters in the first shell were located at an average distance of 2.44Å (2.46Å - 122 water simulation). The second hydration shell was composed of distinct equatorial and apical regions resulting in a peak in the U-O radial distribution function at 4.59Å. The equatorial second shell contained 10 water molecules hydrogen-bonded to the five first shell molecules. Above and below the UO₂²⁺ ion, the water molecules were found to be significantly less structured. In these apical regions, water molecules were found to sporadically hydrogen bond to the oxygen atoms of the UO₂²⁺; oriented in such way as to have their protons pointed towards the cation. While the number of apical waters varied greatly, an average of 5-6 waters was found in this region. Many water transfers into and out of the equatorial and apical second solvation shells were observed to occur on a picosecond (ps) time scale via dissociative mechanisms. Beyond these shells, the bonding pattern substantially returned to the tetrahedral structure of bulk water.
2008. "The Impact of Molecular Interactions on Atmospheric Aerosol Radiative Forcing." Advances in Quantum Chemistry 55:429-447. doi:10.1016/S0065-3276(07)00220-1 Abstract We review the chemical physics of nucleation and its connection to atmospheric aerosol radiative forcing. The scientific community has demanded a comprehensive investigation of aerosol radiative forcing of climate. Particular emphasis has been placed on gaining the fundamental knowledge necessary to accurately predict aerosol formation and growth and their subsequent radiative affects on climate. The chemical physics of the molecular clusters underlying nucleation will be outlined and future developments towards modeling multi-component nucleation in the atmosphere discussed. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) and calculations were performed in part using the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) at the Pacific Northwest National Laboratory. The EMSL is funded by the DOE Office of Biological and Environmental Research. Battelle operates Pacific Northwest National Laboratory for DOE.
2008. "On the Determination of Monomer Dissociation Energies of Small Water Clusters from Photoionization Experiments." Journal of Physical Chemistry A 112(9):1851-1853. doi:10.1021/jp710624r Abstract Recently, water monomer dissociation energies from neutral water clusters were estimated from the measured appearance energies resulting from vacuum ultraviolet photoionization. The monomer dissociation energies of neutral water clusters were determined via a thermodynamic cycle, which encompassed the experimentally measured appearance energies of the photoionized water clusters and the previously reported dissociation energies of protonated water clusters. A key approximation was assumed - that the relaxation energy for the process ! (H2O)n + "(H2O)n -1H+ +OH• is zero. We will show that the relaxation energies are large and thus cannot be neglected. Thus, the neutral water cluster monomer dissociation energies cannot be determined directly from the measured ionization potentials since they are themselves involved in the thermodynamic cycle. This work was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, US Department of Energy. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy. Computer resources were provided by the Office of Science, US Department of Energy.
2008. "Activation Energies and Potentials of Mean Force for Water Cluster Evaporation." Journal of Chemical Physics 128(6):Art. No. 064306. doi:10.1063/1.2837282 Abstract Activation energies for water cluster evaporation are of interest in many areas of chemical physics. We present the first computation of activation energies for small waters clusters using the formalism of Dynamical Nucleation Theory (DNT). To this end, individual evaporation rate constants are computed for water clusters (H2O)i, where i = 2 to 10 for temperatures ranging from 243 to 333K. These calculations employ a parallel sampling technique utilizing the Global Arrays Toolkit developed at PNNL. The resulting evaporation rate constants for each cluster are then fit to Arrhenius equations to obtain activation energies. We discuss DNT evaporation rate constants and their relation to potentials of mean force, activation energies, and how to account for non-separability of the reaction coordinate in the reactant state partition function. This work was supported by the PNNL Computational Science and Engineering LDRD Program and the Chemical and Material Sciences Division, Office of Basic Energy Sciences, Department of Energy. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2008. "Spectroscopic Studies of the Phase Transition in Ammonia Borane: Raman spectroscopy of single crystal NH3BH3 as a function of temperature from 88 to 330 K." Journal of Chemical Physics 128(3):Art. No. 034508. doi:10.1063/1.2820768 Abstract Raman spectra of single crystal ammonia borane, NH3BH3, were recorded as a function of temperature from 77 to 300 K using Raman microscopy and a variable temperature stage. The orthorhombic to orientationally disordered tetragonal phase transition at 225 K was clearly evident from the decrease in the number of vibrational modes. However some of the modes in the orthorhombic phase appeared to merge 10 to 12 K below the phase transition perhaps suggesting the presence of an intermediate phase. Factor group analysis of vibrational spectra for both orthorhombic and tetragonal phase is provided. To our knowledge this is first reported vibrational spectra in the BH and NH stretching region of single crystal NH3BH3 in the orthorhombic phase.
2008. "Electron-Driven Acid-Base Chemistry: Proton Transfer from Hydrogen Chloride to Ammonia." Science 319(5865):936-939. doi:10.1126/science.1151614 Abstract It is well established that NH3 and HCl form in isolation a hydrogen bonded complex NH3…HCl rather than an ionic salt, NH4+Cl-. This experimental and theoretical study utilized anion photoelectron spectroscopy and ab initio theory to investigate the effect of an excess electron on the hydrogen bonded complex NH3 …HCl. Our results indicate that one electron is sufficient to drive the hydrogen bonded complex to form the ionic salt. We propose a stepwise mechanism for this process involving an initial dipole-bound state, followed by the formation of a distorted Rydberg species, NH40.
2008. "Many-Body Decomposition of the Binding Energies for OH•(H2O)2 and OH•(H2O)3 Complexes." Journal of Chemical Physics 128(8):Art. No. 084307. doi:10.1063/1.2828522 Abstract We use ab initio electronic structure methods to calculate the many-body decomposition of the binding energies of the OH(H2O)n (n=2,3) complexes. We employ MP2 and CCSD(T) levels of theory with aug-cc-pVDZ and aug-cc-pVTZ basis sets and analyze the significance of the non-pairwise interactions between OH radical and the surrounding water molecules. We also evaluate the accuracy of our newly developed potential function, the modified Thole-type model (mTTM), for predicting the many-body terms in these complexes. Our analysis of the many-body contributions to the OH(H2O)n binding energies clearly shows that they are just as important in the OH interactions with water as they are for interactions in pure water systems. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) and was performed in part using the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) at the Pacific Northwest National Laboratory. The EMSL is funded by the DOE Office of Biological and Environmental Research. Battelle operates Pacific Northwest National Laboratory for DOE. The authors thank Sotiris Xantheas, Jun Li, Tzvetelin Iordanov, and Jun Cui for helpful discussions and assistance.
2008. "Molecular Structure and Dynamics in the Low Temperature (Orthorhombic) Phase of NH3BH3." Journal of Physical Chemistry A 112(18):4277-4283. doi:10.1021/jp711696 Abstract Variable temperature 2H NMR experiments on the orthorhombic phase of selectively deuterated NH3BH3 spanning the static to fast exchange limits of the borane and amine motions are reported. New values of the electric field gradient (EFG) tensor parameters have been obtained from the static 2H spectra of Vzz = 5.509(±0.275)×1014 statvolt/cm2 and ! = 0.00±0.05 for the borane hydrogens and Vzz = 9.615(±0.481)×1014 statvolt/cm2 and ! = 0.00±0.05 for the amine hydrogens. The molecular symmetry inferred from the observation of equal EFG tensors for both the boron and amine hydrogens is in sharp contrast with the Cs symmetry derived from diffraction studies. The origin of the apparent discrepancy has been investigated using molecular dynamics methods in combination with electronic structure calculations of NMR parameters, bond lengths, and bond angles. The computation of parameters from a statistical ensemble rather than from a single set of atomic Cartesian coordinates gives values that are in close quantitative agreement with the 2H NMR electric field gradient tensor measurements and are more consistent with the molecular symmetry revealed by the NMR spectra. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.
2008. "Self-consistent polarization neglect of diatomic differential overlap: Application to water clusters." Journal of Chemical Physics 128(16):119-137 (Art. no. 164111). doi:10.1063/1.2905230 Abstract Semiempirical SCF methods such as MNDO, AM1, and PM3 have the ability to treat the formation and breaking of chemical bonds but have been found to poorly describe hydrogen bonding and weak electrostatic complexes. In contrast, most empirical potentials are not able to describe bond-breaking and formation, but have the ability to add missing elements of hydrogen bonding using classical electrostatic interactions. We present a new method which combines aspects of both NDDO-based SCF techniques and classical descriptions of polarization to describe the diffuse nature of the electronic wavefunction in a self-consistent manner. We develop the “self-consistent polarization neglect of differential diatomic overlap” (SCP-NDDO) theory with the additional description of molecular dispersion developed as a second-order perturbation theory expression. The current study seeks to model water-water interactions as a test case. To this end, we have parameterized the SCP-NDDO model to the accurate MP2/CBS estimates of small water cluster binding energies of Xantheas et al.[S. S. Xantheas, C. J. Burnham, and R. J. Harrison, J. Chem. Phys. 116, 1493 (2002); S. S. Xantheas and E. Aprà, J. Chem. Phys. 120, 823 (2004)]. Overall agreement with the ab initio binding energies (n = 2 – 6, 8) is achieved with an RMS error of 0.20 kcal/mol. We achieve noticeable improvements in the structure, vibrational frequencies, and energetic predictions of water clusters (n ≤ 21) relative to standard NDDO-based methods.
2007. "Hybrid Approach for Free Energy Calculations with High-Level Methods: Application to the S(N)2 Reaction of CHCl3 and OH- in Water." Journal of Chemical Physics 127(5):51102 (1-4). Abstract We present an approach for potential of mean force calculations within quantum mechanical molecular mechanics framework using hierarchy of approximations involving density functional and high level coupled cluster theories. As an example application we study the reaction process of CHC1₃ with OH¯ in aqueous solution.
2007. "A Special Brew." Natural History 116(9):32-36. Abstract Water is important and ubiquitous and surprisingly not understood. Just because is it common, does not mean its understood "Poets say science takes away from the beauty of the stars-mere globs of gas atoms. I too can see the stars on a desert night, and feel them. But do I see less or more? ... What is the pattern, or the meaning, or the why? It does not do harm to the mystery to know a little about it. For far more marvelous is the truth than any artists of the past imagined it." - Richard P. Feynman, The Feynman Lectures on Physics, 1963. (Cited in the introduction to Chapter 3 of "The Snowflake, Winter's Secret Beauty, Text by Kenneth Libbrecht, Photography by Patricia Rasmussen.) 1. Highlight the fact that water is still one of the most active and challenging research areas in chemistry and physics 2. Describe in general terms why water is unique from the point of view of its properties o Large dipole-moment o Very polarizable o Involved in is own chemistry (e.g. auto ionization defining the pH scale) • Atomic view: o Oxygen and Hydrogen. o Hydrogen is a quantum mechanical in nature. Classical physics is no good. o Water’s Charge-charge interaction described by classical physics laws (e.g. Coulomb) o The statistical mechanics of water. Why counting is important. o You need the full arsenal of theoretical methods to understand water • Waters well known bulk properties do not explain waters anomalies o Surface tension, heat capacity • Understanding the microscopic nature of water and how this gives rise to the known bulk quantities is the thrust of state-of-the-art research o Hydrogen bonding o Liquid structure o The so-called “spherical cow” model gets you no where with water o There are 10s-100s of different water models available in the scientific literature. It is a hard business • All of life takes place at the interfaces of solid, liquid, and gas o Biology takes advantage of waters varying properties in different geometries (e.g. confined, surfaces, etc. o Water behaves differently in confined environments • Water is the most abundant greenhouse gas o How does a microscopic understanding of water impact our knowledge of the radiation budget of the earth o How does a microscopic understanding of water impact our knowledge of weather Nucleation Condensation Evaporation This work was supported by the Office of Basic Energy Sciences of the Department of Energy. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.
2007. "Isomers and Conformers of H(NH₂BH₂)(n)H Oligomers: Understanding the Geometries and Electronic Structure of Boron-Nitrogen-Hydrogen Compounds as Potential Hydrogen Storage Materials." Journal of Physical Chemistry C 111(8):3294-3299. doi:10.1021/jp066360b Abstract Boron-nitrogen-hydrogen (BNHx) materials are polar analogs of hydrocarbons with potential applications as media for hydrogen storage. As H(NH₂BH₂)nH oligomers result from dehydrogenation of NH₃BH₃ and NH₄BH₄ materials, understanding the geometries, stabilities, and electronic structure of these oligomers is essential for developing chemical methods of hydrogen release and regeneration of the BNHx-based hydrogen storage materials. In this work we have performed computational modeling on the H(NH₂BH₂)nH (n = 1 – 6) oligomers using density functional theory (DFT). We have investigated linear chain structures and the stabilizing effects of coiling, biradicalization, and branching through Car-Parrinello molecular dynamics simulations and geometry optimizations. We find that the zig-zag linear oligomers are unstable with respect to the coiled, square-wave chain, and branched structures, with the coiled structures being the most stable. Dihydrogen bonding in oligomers, where protic Hδ⁺(N) hydrogens interact with hydridic Hδ⁻(B) hydrogens, plays a crucial role in stabilizing different isomers and conformers. The results are consistent with structures of products that are seen in experimental NMR studies of dehydrogenated ammonia borane.
2007. "The Critical Role of Anharmonicity in Aqueous Ionic Clusters Relevant to Nucleation." Journal of Physical Chemistry C 111(13):4977-4983. doi:10.1021/jp067468u Abstract Calculating accurate free energies of aqueous ionic clusters is important in many areas of chemical physics. A common practice used in ab initio and other methods to determine the cluster free energy is to use a single energetically favorable configuration and then assume the harmonic approximation. In the present study, we compare and contrast harmonic free energies to those determined from anharmonic configurational sampling and how both compare to experiment. It is found that anharmoncity plays a critical role in determining accurate free energies for the clusters underlying ion-induced nucleation. Consequently, energies, geometries, and frequencies obtained using a single or multiple configurations within the harmonic approximation lead to incorrect aqueous cluster thermodynamics.This work was supported by the Office of Basic Energy Sciences of the Department of Energy, in part by the Chemical Sciences Program. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2007. "Comment on "Quantum Nature of the Sign Preference in Ion-Induced Nucleation"." Physical Review Letters 98(10):Art.no.109603. Abstract A reliable molecular-level treatment of nucleation requires (i) accurate descriptions of the interaction potentials, (ii) a consistent theoretical approach that connects the interaction potential to the nucleation rate, and (iii) appropriate statistical mechanics to evaluate quantities of interest. In a recent PRL Nadykto et al. [1] “attack the fundamental problem of the sign preference by employing a higher level of theory such as quantum mechanics”, where quantum mechanics in this context refers to electronic structure calculations of interaction energies. They concluded that “The strong effect of the chemical nature of the core ion on the conversion of vapor molecules to clusters is essentially quantum in nature, and, thus, systematic accounting for the actual core species is impossible without taking into account the actual electronic structures of the core ions.” The purpose of this Comment is to point out conceptual errors concerning the critical importance of items (ii) and (iii) above and to clarify misrepresentations of our previous work on this topic [2]. Any consistent theory of nucleation must describe the relevant regions of configuration space that govern the kinetics and thermodynamics of cluster formation through evaporation and condensation processes – e.g., Dynamical Nucleation Theory (DNT) [3].
2007. "Ab initio and analytical intermolecular potential for ClO-H2O." Journal of Chemical Physics 126(11):146-155. doi:10.1063/1.2566537 Abstract In recent years, the ClO free radical has been found to play an important role in the ozone removal processes in the atmosphere. In this work, we present a Potential Energy Surface (PES) Scan of the ClO•H2O system with high-level ab initio methods. Because of the existence of low-lying excited states of the ClO•H2O complex, and their potential impact on the chemical behavior of the ClO radical in the atmosphere, we perform a PES scan at CCSD(T)/aug-cc-pVTZ level of both the first excited and ground states in order to model the physics of the unpaired electron in the ClO radical. Analytical potentials for both ground and excited states, with internal molecular coordinates held fixed, were built based on a Thole Type Model. The two minima of the ClO•H2O complex are recovered by the analytical potential. This work was supported by the Office of Basic Energy Sciences of the Department of Energy, in part by the Chemical Sciences program and in part by the Engineering and Geosciences Division. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.
2006. "Critical comparison of classical and quantum mechanical treatments of the phase equilibria of water ." Journal of Chemical Physics 124:114505. doi:10.1063/1.2178322 Abstract The Gibbs ensemble Monte Carlo simulation technique was used to compare the phase equilibria of the rigid TIP4P water model [J.\ Chem.\ Phys.\ {\bf 79}, 926, (1983)] utilizing classical and quantum statistical mechanical treatments. The quantum statistical mechanical treatment generally resulted in lower liquid densities and higher vapor densities, showing a narrowing in the phase envelope. As a result, the calculated critical temperature and normal boiling point were higher from the quantum simulations than the classical by 20~and 17~K, respectively, but the critical densities were very close. A semiclassical treatment, involving a low order expansion in Plank's constant, resulted in densities and vapor pressures that fluctuated between the values for the classical and quantum statistical mechanics values, with no definite agreement with either.
2006. "Sensitivity analysis of thermodynamic properties of liquid water: A General Approach to Improve Empirical Potentials ." Journal of Physical Chemistry A 110(2):762-771. Abstract A sensitivity analysis of bulk water thermodynamics is presented in an effort to understand the relation between details of molecular potentials and the properties that they predict. The analysis is incorporated in molecular dynamics simulation and investigates the sensitivity of the Helmholtz free energy, internal energy, entropy, heat capacity, pressure, thermal pressure coefficient, and static dielectric constant to details of the potential. The sensitivities of the properties are calculated with respect to the van der Waals repulsive and the attractive parts, plus short and long range Coulomb parts of three four site empirical water potentials: TIP4P, Dang-Chang and TTM2R. The polarization sensitivity is calculated for the polarizable Dang-Chang and TTM2R potentials. The analysis indicates that all investigated properties are most sensitive to the van der Waals repulsive, the short range Coulomb and the polarization components of the potentials. The sensitivity of the Helmoltz free energy, internal energy, and entropy due to polarizaion is almost 30% of total electrostatic sensitivity. In addition the similarities in the trends of the observed sensitivities for nonpolarizable and polarizable potentials lead to the conclusion that the complexity of the model is not of critical importance for the calculation of these properties for bulk water. The van der Waals attractive and the long range Coulomb sensitivities are relatively small for the entropy, heat capacity, thermal pressure coefficient and the static dielectric constant, while small changes in any of the potential contributions will significantly effect the pressure. The analysis suggests a procedure for modification of the potentials and their improved prediction of thermodynamic properties. Based on the proposed procedure the water cluster potential TTM2R was adapted for simulation of bulk water properties.
2006. "Electronic Structure, Statistical Mechanical Simulations, and EXAFS Spectroscopy of Aqueous Potassium." Theoretical Chemistry Accounts 115(2-3):86-99. doi:10.1007/s00214-005-0054-4 Abstract We investigate the solvation structure of aqueous potassium ions, using a combination of electronic structure calculations, statistical mechanical simulations with a derived polarizable empirical potential and experimental measurement of the Extended X-ray Absorption Fine Structure (EXAFS) spectra. The potassium K-edge (at 3608 eV) EXAFS spectra were acquired on the bending magnet of sector 20 at the Advanced Photon source, at ambient conditions and for the concentrations of 1m and 4m KCl. We focus on the coordination distances and the degree of disorder of the first hydration shell as determined by electronic structure calculations, molecular dynamics simulations and experimental measurement. Finally, we characterize the changes of the structure in the first hydration shell with increasing temperature as predicted by molecular simulation.
2006. "Molecular simulations of the transport of molecules across the liquid/vapor interface of water." Chemical Reviews 106(4):1355-1374. Abstract The transport of molecules across the liquid/vapor interface of water is a fundamental process that is important in a number of areas. Uptake of trace molecules by aqueous droplets is important in the atmosphere as it enables heterogeneous new chemical pathways that are not available in the gas phase. The uptake of molecules by water and partitioning to the air/water surface is important in aquatic environmental systems. The summary of molecular simulations presented in this work supports a picture of solute transport across the water’s vapor/liquid interface in which a hydrophilic solute molecule impinging on the surface is rapidly equilibrated, sticks to the surface of the interface with nearly unit probability, then diffuses into the bulk liquid on an free energy surface. Analysis of a large number of experimental observations of uptake, using a variety of techniques, supports a view of mass transport that is diametrically opposed – most solute molecules that collide with the surface return to the vapor rather than being absorbed into the liquid. This discrepancy stresses the need for greater understanding of the important process of uptake. We provide a perspective on molecular-scale simulations and evaluate the type of accuracy we should expect for interfacial properties, particularly those associated with the update of molecular at the surface. We briefly review computational methods commonly used in studies of aqueous interfaces, including functional forms of the molecular interaction potentials, descriptions of the simulation methodologies, and descriptions of the molecular models of the interface. We present the results of simulations of select properties of the air/water interface and properties of molecular interactions at interfaces and present an overview of modeling approaches to the macroscopic process of uptake. In particular, we present detailed descriptions of the uptake process in flow tube experiments with the help of fluid dynamics calculations toward critical comparison between the uptake experiments and the molecular simulations.
2006. "A quantitative account of quantum effects in liquid water." Journal of Chemical Physics 125(14 ):Art. No. 141102. Abstract We report quantum statistical mechanical simulations of liquid water with the TTM2.1-F flexible, polarizable interaction potential for water. The potential is the first representation of the molecular interaction that reproduces the converged Born-Oppenheimer potential energy surface obtained from systematically improvable electronic structure analysis of binding energies of water clusters. Proper quantum statistical simulation of properties allows for a quantitative account of the magnitude of quantum effects in liquid water. We report path integral quantum dynamical simulations of total length of 600 ps with a 0.05 fs time step for a periodic system of 256 molecules. The representation of the quantum effects was achieved using up to 32 replicas per atom. These allow for a quantitative description of the broadening of the radial distribution functions and the corresponding energy shifts in the heat of vaporization. Our best estimate for the enthalpy of the liquid from the results of the quantum simulations is in the range 10.4 – 10.6 kcal/mol, in agreement with the experimental value of 10.51 kcal/mol. 1Battelle operates PNNL for the USDOE
2006. "The OH Radical-H2O Molecular Interaction Potential." Journal of Chemical Physics 124:224318 (15). doi:10.1063/1.2200701 Abstract The OH radical is one of the most important oxidants in the atmosphere due to its high reactivity. The study of hydrogen-bonded complexes of OH with the water molecules is a topic of significant current interest. In this work, we present the development of a new analytical functional form for the interaction potential between rigid OH radical and H2O molecules. To do this we fit a selected functional form to a set of high level ab initio data. Since there are low-lying excited states for the H2O•OH complex, the impact of the excited states on the chemical behavior of the OH radical can be very important. We perform a Potential Energy Surface (PES) scan using the CCSD(T)/aug-cc-pVTZ level of electronic structure theory for both excited and ground states. To model the physics of the unpaired electron in the OH radical, we develop a tensor polarizability generalization of the Thole Type all-atom polarizable rigid potential for the OH radical, which effectively describes the interaction of OH with H2O for both ground and excited states. The stationary points of (H2O)n•OH clusters were identified as a benchmark of the potential. Battelle operated PNNL for the USDOE.
2006. "Molecular simulation analysis and X-ray absorption measurement of Ca2+, K+, and Cl- ions in solution." Journal of Physical Chemistry B 110(47):23644-23654. Abstract Recent advances in the use of molecular simulations and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy to understand solvated ions in aqueous solutions are described. We report and discuss the results of the EXAFS spectra, Debye-Waller factors and the related properties governing solvation processes of different ions in water, as well as in different solvents (methanol). Molecular dynamics (MD) trajectories are coupled to electron scattering simulations to generate the EXAFS spectra, which are found to be in very good agreement with the corresponding experimental measurements. From these spectra, both the ion-oxygen and the ion-hydrogen distances for the first hydration shell are predicted to be within 0.1-0.2 Å. The ionic species studied range from monovalent to divalent, positive and negative: K+, Ca2+ and Cl-. This work demonstrates that the combination of MD-EXAFS and the corresponding experiment measurement provides a powerful tool in the analysis of the solvation structure of aqueous ionic solutions. We also investigate the value of electronic structure analysis of small aqueous clusters as a benchmark to the empirical potentials. In a novel computational approach, we compute the Debye-Waller factors combining a harmonic analysis of data obtained from electronic structure calculations on finite ionwater clusters, and we present a direct comparison with results from a harmonic classical statistical mechanical analysis of an empirical potential. Work was supported by the Office of Science, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy (DOE). The Pacific Northwest National Laboratory is operated by Battelle for DOE.
2005. "Ion-Induced Nucleation: The Importance of Chemistry." Physical Review Letters 94(11):116104. Abstract Experiments have shown that ions can substantially increase vapor-to-liquid nucleation rates. However, interpretation of these experiments is complicated by ambiguities arising from the manner in which the ions are produced. Several studies have concluded that water has a general preference for anions over cations. We show that specification of ion’s sign alone is insufficient to provide an understanding of the aqueous ionic cluster thermodynamics and that Classical Ion-Induced Nucleation Theory does not treat the cluster physics properly to describe ion-induced nucleation accurately.
2005. "Role of Water in Electron-Initiated Processes and Radical Chemistry: Issues and Scientific Advances." Chemical Reviews 105(1):355-389. doi:10.1021/cr030453x Abstract An understanding of electron-initiated processes in aqueous systems and the subsequent radical chemistry these processes induce is significant in such diverse fields as waste remediation and environmental cleanup, radiation processing, nuclear reactors, and medical diagnosis and therapy. We review the state of the art in the physical chemistry and chemical physics of electron-initiated processes in aqueous systems and raise critical research issues and fundamental questions that remain unanswered.
2004. "Multicomponent Dynamical Nucleation Theory And Sensitivity Analysis ." Journal of Chemical Physics 120(19):9133-9141. Abstract Vapor to liquid multi-component nucleation is a dynamical process governed by a delicate interplay between condensation and evaporation. Since the population of the vapor phase is dominated by monomers at reasonable supersaturations, the formation of clusters is governed by monomer association and dissociation reactions. Since no potential energy barrier exists the formation of a cluster is impeded by a free energy of activation that is entropic in nature. Dynamical Nucleation Theory provides a framework in which equilibrium evaporation rate constants can be calculated and the corresponding condensation rate constants determined from detailed balance. The nucleation rate can then be obtained by solving the kinetic equations. The rate constants governing the multi-step kinetics of multi-component nucleation including sensitivity analysis and the influence of possible contaminants will be presented and discussed.
2004. "Intermolecular potential and second virial coefficient of the water-hydrogen complex." Journal of Chemical Physics 120(2):710-720. Abstract We construct a rigid-body (5-dimensional) potential-energy surface for the water–hydrogen complex using scaled perturbation theory (SPT). An analytic fit of this surface is obtained, and using this, two minima are found: the global minimum has C2v symmetry, with the hydrogen molecule acting as a proton donor to the oxygen atom on water; a local minimum with Cs symmetry has the hydrogen molecule acting as a proton acceptor to one of the hydrogen atoms on water, where the OH bond and H2 are in a T-shaped configuration. The SPT global and local minima are bound by 241 and 142 cm
2004. "Helium Diffusion Through H2O and D2O Amorphous Ice: A Lattice Inverse Istope Effect." Physical Review Letters 92(19):Art. no. 198306. Abstract The diffusion of helium through both H2O and D2O amorphous solid water (ASW) has been measured between 55K and 110K. We find the diffusion rate is dependent on the isotopic composition of the ASW lattice. This lattice isotope effect is the "inverse" of a normal isotope effect, in that diffusion is faster in the heavier (D2O) isotope. Transition state theory calculations show that the isotope effect is due to a tight transition state results in a large zero point vibrational energy differences at the transition state predominantly due to hindered rotations of water in the lattice.
2003. "Thermochemistry and kinetics of evaporation and condensation for small water clusters." Chapter 2 in Water in Confining Geometries, ed. V. Buch and J. P. Devlin, pp. 25-51. Springer-Verlag, New York, NY. Abstract The evaluation of thermochemical properties of small water clusters (e.g., consisting of 2 - 10 water molecules) is complicated by their dissociative nature. At temperatures for which dissociation of the cluster into molecular fragments has significant probability, an "operational definition" of a cluster is required to restrict the phase space and evaluate a finite value for the partition function or cluster property of interest. We will review a theoretical approach, Dynamical Nucleation Theory (DNT), for evaluating rate constants for cluster evaporation and condensation, which allows for a unique definition of clusters and their thermochemical properties. We will also discuss the relevance of DNT to homogeneous vapor-to-liquid nucleation of water.
2003. "Probing Nanoscale Surface Enhanced Raman Scattering Fluctuation Dynamics using Correalted AFM and Confocal Ultramicroscopy." Ultramicroscopy 97(1-4):89. Abstract We have studied the laser-excitation-intensity-dependent and Ag-nanocluster interstitial-site-dependent SERS intensity fluctuations under low molecule surface coverage of rhodamine 6G and cytochrome c. a new two-channel photon time-stamping system coupled with atomic force microscopic (AFM), Raman spectroscopic, and imaging microscopy was developed and applied to record Raman intensity fluctuation trajectories at sub-microsecond resolution correlated with in-situ characterization of the nanoparticle clusters. Our experimental results suggest that the nanoconfinement of the local electromagnetic-field enhancement and the interaction of the local field with the molecules, presumably under rotational motions, result in nano-Raman fluctuations. The SERS spectral fluctuation was pertinent to the nanoscale local enhancement and local interaction of the molecules with the surface when the number of molecules to contribute the microscopic Raman signal collected from a diffraction-limited focus spot. The SERS fluctuation dynamics were both photo-induced and spontaneous for rhodamine 6G, but only the photo-induced interstitial sites with heterogeneous geometries. To interpret the observed nano-SERS fluctuation dynamics, we used computer simulation of optical multiple scattering, based on multi-sphere scattering Mie theory, and rotational diffusion of molecules at an interstitial site, based on a random walk in orientation space.
2003. "Generalized Transition State Theory in Terms of the Potential of Mean Force." Journal of Chemical Physics 119(12):5828-5833. doi:10.1063/1.1597477 Abstract The relationship between the free energy of activation and the potential of mean force is derived for reaction coordinates that are arbitrary functions of all the coordinates defining a system. The general result is illustrated for rectilinear reaction coordinates, bond distance reaction coordinates, atom transfer reaction coordinates, synchronous double atom transfer reaction coordinates, and energy gap reaction coordinates.
2003. "Potentials of Mean Force With Ab Initio Mixed Hamiltonian Models of Solvation ." Journal of Molecular Structure - Theochem 632(1-3):173-183. Abstract We give an account of a computationally tractable and efficient procedure for the calculation of potentials of mean force using mixed Hamiltonian models of electronic structure where quantum subsystems are described with computationally intensive ab initio wavefunctions. The mixed Hamiltonian is mapped into an all-classical Hamiltonian that is amenable to a thermodynamic perturbation treatment for the calculation of free energies. A small number of statistically uncorrelated (solute-solvent) configurations are selected from the Monte Carlo random walk generated with the all-classical Hamiltonian approximation. Those are used in the averaging of the free energy using the mixed quantum/classical Hamiltonian. The methodology is illustrated for the micro-solvated SN2 substitution reaction of methyl chloride by hydroxide. We also compare the potential of mean force calculated with the above protocol with an approximate formalism, one in which the potential of mean force calculated with the all-classical Hamiltonian is simply added to the energy of the isolated (non-solvated) solute along the reaction path. Interestingly the latter approach is found to be in semi-quantitative agreement with the full mixed Hamiltonian approximation.
2003. "EXAFS spectra of the dilute solutions of Ca2+ and Sr2+ in water and methanol ." Journal of Physical Chemistry B 107(50):14119-14123. Abstract A set of polarizable ion-water intermolecular interactions were developed that accurately described solvation enthalpies and structural properties of the dilute Ca2+ and Sr2+ in aqueous solution. The molecular dynamics (MD), coupled to electron scattering simulations of the Ca2+ and Sr2+ X-ray absorption fine structure spectroscopy (EXAFS) spectra are in good agreement with the corresponding experimental measurements. This work demonstrated that the combination of MD-EXAFS and the corresponding experiment measurement provides a power tool in the analysis of the solvation structure of the aqueous ionic solutions. The Ca2+-methanol interaction was also developed and the dilute Ca2+ MD-EXAFS spectrum in liquid methanol was also predicted using same approaches.
2002. "The Development of Effective Classical Potentials and the Quantum Statistical Mechanical Second Virial Coefficient of Water." Journal of Chemical Physics 117(14):6573-6581. Abstract The second virial coefficient of water is calculated at low temperature by considering full quantum statistical mechanical effects. At low enough temperatures experimental results are limited and molecular models can be used for accurate extrapolation. In doing so, one must separate inaccuracies of the intermolecular potential from limitations of simulation such as the neglect of higher-order quantum corrections. Effective classical potentials may be used to understand the limitations of classical simulation. In this work we calculate the exact quantum statistical mechanical second virial coefficient and find that using a simple form for the effective classical potential introduced by Miller we are able to reproduce the exact quantum statistical results. This approach provides a significant improvement to conventional first order expansions of the second virial coefficient.
2002. "Equilibrium Constant for Water Dimerization: Analysis of the Partition Function for a Weakly Bound System." Journal of Physical Chemistry A 106(8):1557-1566. Abstract The treatment of dissociative states in the calculation of the partition function of a weakly bound system, such as the water dimer, is discussed. For a dissociative system, the number of phase-space configurations that contribute to the total partition function from energies above the dissociation energy depend upon the system volume. For a sufficiently large system volume, entropy from these confiurations will dominate over the energy contribution of the local minimum and contributions from dissociative states will dominate the total partition function. The calculation of the dimer partition function requires limiting the phase space of the cluster or providing a definition of those phase space points that correspond to a dimer. Since there is no unique procedure to constrain the phase space of a dimer, we provide an analysis of the dimer partition function using different constraints. For the water dimer at temperatures in the range 200-500 K, the value of the dimer partition function changes by up to 3 orders of magnitude with variations of the constraint.
2002. "Dynamical benchmarks of the nucleation kinetics of water." Journal of Chemical Physics 116(10):4275-4280. Abstract Recently a theory of vapor-to-liquid phase nucleation was developed that was based on the kinetics of cluster formation and decomposition. The new method used variational transition state theory (VTST) to obtain the evaporation and condensa-tion rate constants needed in the kinetic model of nucleation. VTST provides a means to systematically improve estimates for rate constants involved in the nucleation process. In the current work we perform dynamical simulations of the condensation process, estimating the effective reactive cross-section using a definition of a cluster that is determined from VTST. These calculations allow us to characterize dynamical corrections to the VTST rate constants. We find that for water cluster sizes rang-ing from 10-40 waters, VTST estimates of the condensation and evaporation rate constants using a spherical dividing surface require dynamical corrections that are approximately a factor of two.
2002. "Understanding the Sensitivity of Nucleation Kinetics: A Case Study on Water." Journal of Chemical Physics 116(12):5046-5057. Abstract Small atomic or molecular clusters provide the bridge between vapor and liquid phases. Nucleation is a rare event process by which clusters of a new phase are produced. This process is inherently dynamic and as such the new phase cannot exist until an activation barrier is surmounted. Dynamical Nucleation Theory (DNT) utilizes variational transition state theory (VTST) to provide a framework in which cluster evaporation and condensation rate constants can be determined directly. To date, the fundamental nature regarding the intrinsic instability of the kinetics of the nucleation process has eluded theoretical efforts. In this paper we present a sensitivity analysis of the homogeneous nucleation rate on kinetic parameters used in DNT. Moreover, several classical interaction potentials for water exist; most of which have been parameterized to reproduce some bulk properties of water at ambient conditions. Thus, an analysis was undertaken to explore what effects the different water potentials have on the dynamical quantities relevant to nucleation. The implication of these results on the direction of future work will be discussed.
2001. "Sculpting the Oil-Water Interface to Probe Ion Solvation." Journal of Physical Chemistry B 105(13):2483-2498. Abstract Solvation of ions at oil-water interfaces is so important in cell wall and enzyme function, colloidal chemistry, fuel cells and other areas, that substantial effort has been made to understand the process via molecular-scale simulations of well-specified model systems. Here we report a series of experiments probing ion transport and solvation in composite films that have a geometrical specificity that rivals what theory is routinely able to employ. Proton/hydronium transport across the water-organic interface has been studied using a novel "soft-landed" ion technique that allows precision tailoring of the interfaces (including initial ion position) and sensitive monitoring of the ion motion via the electric potential generated by the ions. There are two main findings. First, the ion solvation by water at the organic/aqueous interface is probed continuously for various monolayers of water present in simple and complex sandwiched structures. The potential trap created by the solvation can be systematically overwhelmed by the collective electric field of the ions, giving us some unique information about both the trap depth and shape. A simple Born model for the trap reproduces some, but not all the details experimentally observed. Second, ion transport in several organic glasses is well predictable by continuum viscosity models at electric fields up to approximately 108 V/m.
2001. "Variational transition state theory evaluation of the rate constant for proton transfer in a polar solvent." Journal of Chemical Physics 115:8460-8480. Abstract N/A
2000. "The Quantum Vibrational Dynamics of Cl-(H2O)n Clusters." Journal of Chemical Physics 113(13):5171-5178. Abstract The centroid molecular dynamics technique is applied to the case of chloride- water clusters to estimate their finite temperature quantum vibrational structure. We employ the flexible RWK2 water potential [J.R. Reimers, R.O. Watts, and M.L. Klein, Chem. Phys. 64, 95 (1982)] and the parametrization of a chloride-water interaction potential of Dorsett, Watts and Xantheas.[J. Phys. Chem. A 103, 3351 (1999)] We then investigate the temperature-dependent vibrational structure (infrared spectra). We find that the centroid molecular dynamics technique is capable of recovering a majority of the redshift associated with hydrogen bonding.
1999. "Stimulated Desorption by Surface Electron Standing Waves." Physical Review Letters 82(16):3348-3351. Abstract The total electron-stimulated desorption yield of Cl+ ions from the Cl-terminated Si(111) surface is shown to exhibit fine-structure oscillations as a function of incident electron beam direction...
1999. "Variational Transition State Theory of Vapor Phase Nucleation." Journal of Chemical Physics 110(16):7951-7959.
1999. "Statistical Analyses and Theoretical Models of Single-Molecule Enzymatic Dynamics." Journal of Physical Chemistry A 103:10477-10488. Abstract N/A
1999. "Dynamical Nucleation Theory: A New Molecular Approach to Vapor-Liquid Nucleation." Physical Review Letters 82(17):3484-3487.
1999. "Dynamical Nucleation Theory: Calculation of Condenstion Rate Constants for Small Water Clusters." Journal of Chemical Physics 111(10):4688-4697.
1999. "XAFS Debye-Waller factors in aqueous Cr+3 from molecular dynamics ." Journal of Synchrotron Radiation 6:310-312.
1998. "Comparison of Classical and Quantum Statistical Mechanical Simulations of Aqueous Ionic Clusters." In Advances in Classical Trajectory Methods, Volume 3: Comparisons of classical and quantum dynamics , ed. William L. Hase, pp. 1-33. JAI Press, Inc., Stamford, CT. Abstract There is no abstract currently available for this item