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. "Implementation of Dynamical Nucleation Theory with Quantum Potentials." Journal of Computational Chemistry 30(5):743-749. Abstract A method is implemented within the context of Dynamical Nucleation Theory in order to efficiently determine the ab initio water dimer evaporation rate constant. The drive for increased efficiency in a Monte Carlo methodology is established by the need to use relatively expensive quantum mechanical interaction potentials. A discussion is presented illustrating the theory, algorithm, and implementation of this method to the water dimer. Hartree-Fock and second order Møller Plesset perturbation theories along with the Dang-Chang polarizable classical potential are utilized to determine the ab initio water dimer evaporation rate constant. 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. "High performance computations using dynamical nucleation theory." Journal of Physics: Conference Series 125:012017. doi:10.1088/1742-6596/125/1/012017 Abstract Chemists continue to explore the use of very large computations to perform simulations that describe the molecular level physics of critical challenges in science. In this paper, the Dynamical Nucleation Theory Monte Carlo (DNTMC) model - a model for determining molecular scale nucleation rate constants - and its parallel capabilities are described. The potential for bottlenecks and the challenges to running on future petascale or larger resources are delineated. A "master-slave" solution is proposed to scale to the petascale and will be developed in the NWChem software. In addition, mathematical and data analysis challenges are also described. 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. "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. "Electronic Effects on the Surface Potential at the Vapor-Liquid Interface of Water." Journal of the American Chemical Society 130(49):16556-16561. doi:10.1021/ja802851w Abstract The surface potential at the vapor-liquid interface of water is relevant to many areas of chemical physics. We present the first computation of the surface potential of water using ab initio molecular dynamics. We find that the surface potential χ = -18 mV with a maximum interfacial electric field = 8.9 × 107 V/m. A comparison is made between our quantum mechanical results and those from previous molecular simulations. We find that explicit treatment of the electronic density makes a dramatic contribution to the electric properties of the vapor-liquid interface of water. The E-field can alter interfacial reactivity and transport while the surface potential can be used to determine the “chemical” contribution to the real and electrochemical potentials for ionic transport through the vapor-liquid interface. 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). Pacific Northwest National Laboratory is operated by Battelle for the 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. "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.
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. "The Use of Processor Groups in Molecular Dynamics Simulations to Sample Free-Energy States." Journal of Chemical Theory and Computation 3(2):583-592. doi:10.1021/ct600260u Abstract Molecular dynamics calculations composed of many independent simulations are frequently encountered in free energy simulations, as well as many other simulation approaches. In principal, the availability of a large number of independent tasks should make possible the development of highly scalable parallel code that executes these tasks concurrently. This paper discusses the use of processor groups to write simulation codes of this type and describes results a code that evaluates the volume dependence of the Helmholtz free energy for clusters of an immiscible fluid in a solvent. The results show that very high levels of scalability can be achieved using processor groups with corresponding reductions in the time to completion. The main limitation to scaling appears to be load imbalance due to variations in the execution times of the individual tasks.
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].
2006. "Understanding the Chemical Physics of Nucleation." Theoretical Chemistry Accounts 116(1-3):169-182. doi:10.1007/s00214-005-0018-8 Abstract Observation and theory have steadily progressed our understanding of nucleation phenomena over the past 280 years. However, even more questions remain concerning the governing processes and mechanisms. The inherent instability and sensitivity of nucleation places a high premium on theoretical accuracy and experimental purity and similarly makes interpretation of both more challenging. The objective of the present paper is to contribute to the understanding of nucleation kinetics and thermodynamics with emphasis on cluster chemical physics within the context of Dynamical Nucleation Theory. Our hope is to share some insights that we’ve gained over the past several years concerning rate constants, molecular interactions, statistical mechanics and their consequences on nucleation.
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.
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.
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.
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. "Monte Carlo Simulations of Small Sulfuric Acid - Water Clusters." Journal of Physical Chemistry B 105(47):11719-11728. Abstract Effective atom-atom potentials are developed for binary sulfuric acid - water clusters and applied in a Bennett Metropolis Monte Carlo calculation to determine free energy differences for small neighboring sized clusters of fixed composition at 298 K. The atom-atom pair potentials consist of Lennard-Jones short-range and Coulombic long range terms and assume rigid $SO_{4}^{2\delta -}$ with two unconstrained $H^{\delta +}$ and rigid $H_{2}O$ molecules interacting via revised central force ({\sc RSL2}) potentials. The potential parameters are determined from both {\it ab initio} studies and tests of the potential using the statistical mechanical formalism for binary cluster size distributions. In the potential tests fixed composition free energy differences, delta f, for $[H_{2}O]_{km}[H_{2}SO_{4}]_{m}$ clusters are plotted versus $(km+m)^{-1/3}$ and the resulting slope and intercept (in the large cluster regime) are used to extract model dependent binary liquid surface tension and partial vapor pressures at $298K$. The potential parameters are adjusted to obtain approximate agreement with experimental surface tension and partial vapor pressures for $k=1$ and $4$ (57\% and 84\% weight percent $H_{2}SO_{4}$, respectively). The free energy differences for $m\leq 15$ are presented, together with internal cluster energy contributions, snapshots of cluster structure and evidence for onset of the large cluster regime near $m=5$. The long range goals have been to test the free energy difference procedure for studying binary cluster properties and to develop model potentials appropriate for the simulation of small binary clusters at low temperatures characteristic of stratospheric sulfuric acid - water aerosols.
1999. "Variational Transition State Theory of Vapor Phase Nucleation." Journal of Chemical Physics 110(16):7951-7959.
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.