2009. "Gaussian Basis Set and Planewave Relativistic Spin-Orbit Methods in NWChem." Journal of Chemical Theory and Computation 5(3):491-499. doi:10.1021/ct8002892 Abstract Relativistic spin-orbit density functional theory (DFT) methods have been implemented in the molecular Gaussian DFT and pseudopotential plane-wave DFT modules of the NWChem electronic-structure program. The Gaussian basis set implementation is based upon the zeroth-order regular approximation (ZORA) while the planewave implementation uses spin-orbit pseudopotentials that are directly generated from the atomic Dirac-Kohn-Sham wavefunctions or atomic ZORA-Kohn-Sham wavefunctions. Compared to solving the full Dirac equation these methods are computationally efficient, but robust enough for a realistic description of relativistic effects such as spin-orbit splitting, molecular orbital hybridization, and core effects. Both methods have been applied to a variety of small molecules, including I$_{\text{2}}$, IF, HI, Br$_{\text{2}}$, Bi$_{\text{2}}$, AuH, and Au$_{\text{2}}$, using various exchange-correlation functionals. Our results are in good agreement with experiment and previously reported calculations.
2009. "Addition of H2O and O-2 to Acetone and Dimethylsulfoxide Ligated Uranyl(V) Dioxocations." Journal of Physical Chemistry A 113(11):2350-2358. doi:10.1021/jp807651c Abstract Gas-phase complexes of the formula [UO2(lig)]+ (lig=acetone (aco) or dimethylsulfoxide (dmso)) were generated by electrospray ionization (ESI) and studied by tandem ion-trap mass spectrometry to determine the general effect of ligand charge donation on the reactivity of UO2+ with respect to water and dioxygen. The original hypothesis that addition of O2 is enhanced by strong σ-donor ligands bound to UO2+ is supported by results from competitive collision-induced dissociation (CID) experiments, which show near exclusive loss of H2O from [UO2(dmso)(H2O)(O2)]+, while both H2O and O2 are eliminated from the corresponding [UO2(aco)(H2O)(O2)]+ species. Ligand-addition reaction rates were investigated by monitoring precursor and product ion intensities as a function of ion storage time in the ion-trap mass spectrometer: these experiments suggest that the association of dioxygen to the UO2+ complex is enhanced when the more basic dmso ligand was coordinated to the metal complex. Conversely, addition of H2O is favored for the analogous complex ion that contains an aco ligand. Experimental rate measurements are supported by density function theory calculations of relative energies, which show stronger bonds between UO2+ and O2 when dmso is the coordinating ligand, while bonds to H2O are stronger for the aco complex.
2009. "Cerium Oxyhydroxide Clusters: Formation, Structure and Reactivity." Journal of Physical Chemistry A 113(22):6239-6252. doi:10.1021/jp9015432 Abstract Cerium oxyhydroxide cluster anions were produced by irradiating ceric oxide particles using 355 nm laser pulses that were synchronized with pulses of nitrogen gas admitted to the irradiation chamber. The gas pulse stabilized the nascent clusters that are largely anhydrous [CexOy] ions and neutrals. These initially-formed species react with water, principally forming closed-shell (c-s) oxohydroxy species that are described by the general formula [CexOy(OH)z]-. In general, the extent of hydroxylation varies from a value of 3 OH per Ce atom when x = 1 to a value slightly greater than 1 for x > 8. The Ce3 and Ce6 species deviate significantly from this trend: the x = 3 cluster accommodates more hydroxyl moieties compared to neighboring congeners at x = 2 and x = 4. Conversely, the x = 6 cluster is significantly less hydroxylated. Density functional theory (DFT) modeling of the cluster structures show that the hydrated clusters are hydrolyzed, and contain one-to-multiple hydroxide moieties, but not datively bound water. DFT also predicts an energetic preference for formation of highly symmetric structures as the size of the clusters increases. The calculated structures indicate that the ability of the Ce3 oxyhydroxide to accommodate more extensive hydroxylation is due to a more open, hexagonal structure in which the Ce atoms can participate in multiple hydrolysis reactions. Conversely the Ce6 oxyhydroxide has an octahedral structure that is not conducive to hydrolysis. In addition to the c-s clusters, open-shell (o-s) oxyhydroxides and superoxides are also formed, and they become more prominent as the size of the clusters increases, suggesting that the larger ceria clusters have an increased ability to stabilize a non-bonding electron. The overall intensity of the clusters tends to monotonically decrease as the cluster size increases, however this trend is interrupted at Ce13, which is significantly more stable compared to neighboring congeners, suggesting formation of a dehydrated Keggin-type structure.
2008. "Quantum Chemical Calculations of the Cl- + CH3I → CH3Cl + I- Potential Energy Surface." Journal of Physical Chemistry A 113(10):1976-1984. doi:10.1021/jp808146c Abstract Electronic structure theory calculations, using MP2 theory and the DFT functionals OPBE, OLYP, HCTH407, BhandH, and B97-1, were performed to characterize the structures, vibrational frequencies, and energies for stationary points on the Cl- + CH3I → ClCH3 + I- potential energy surface. The aug-cc-pVDZ and aug-cc-pVTZ basis sets, with an effective core potential (ECP) for iodine, were employed. Single-point CCSD(T) calculations were performed to obtain the complete basis set (CBS) limit for the reaction energies. DFT was found to give significantly longer halide ion/carbon atom bond lengths for the ion-dipole complexes and central barrier transition state, than MP2. BhandH, with either the aug-cc-pVDZ and aug-cc-pVTZ basis sets, gives good agreement with the experimental structures for both CH3I and CH3Cl. The frequencies of CH3I and CH3Cl, obtained with the different level of theory and basis sets, are in excellent agreement with experiment. The major difference between the MP2 and DFT frequencies is for the imaginary frequency of the central barrier. Using the aug-cc-pVTZ basis the MP2 value for this frequency ranges from 1.26 - 1.59 times larger than those for the DFT functional. Thus, the MP2 and DFT theories have different PES shapes in the vicinity of the [Cl--CH3--I]- central barrier. The CCSD(T)/CBS energies are in good agreement with experiments for the complexation energies and reaction exothermicity, with a small 1 kcal/mol difference for the latter. The CCSD(T)/CBS central barrier height is lower than values deduced by using statistical theoretical models to fit the Cl- + CH3I → ClCH3 + I- experimental rate constant, which is consistent with the expected non-statistical dynamics for the reaction. The BhandH energies are in overall best agreement with the CCSD(T) values, with a largest difference of only 0.7 kcal/mol.
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. "A QM/MM Approach to Interpreting Zn-67 Solid-State NMR data in Zinc Proteins." Journal of the American Chemical Society 130(19):6224-6230. Abstract We present here a 67Zn solid-state NMR investigation of Zn2+ substituted rubredoxin. The sample has been prepared as both a dry powder and a frozen solution to determine the effects of static disorder on the NMR lineshape. Low temperature experiments have been performed at multiple fields to determine the relative contributions to the NMR lineshape from the electric field gradient and the anisotropic shielding tensors. Finally we present the theoretical interpretation of the experimental results utilizing a combined quantum mechanical molecular mechanics (QM/MM) approach. Theory predicts a sizable contribution from anisotropic shielding as compared with previously examined model systems. This is in good agreement with the experimental data.
2008. "Coupled Cluster Calculations for Static and Dynamic Polarizabilities of C₆₀." Journal of Chemical Physics 129(22):226101. doi:10.1063/1.3028541 Abstract New theoretical predictions of the static and frequency dependent polarizabilities of C₆₀ are reported. Using the linear response coupled cluster approach with singles and doubles (LR-CCSD) and a basis set specially designed to treat the molecular properties in external electric field we obtained 82.20ų and 83.62 ų for static and dynamic (λ = 1064nm) polarizabilities. These numbers are in a good agreement with experimentally inferred data of 76.5±8 ų znc 79±4 ų, [R/ Antoine, dt al., J. Chem. Phys. 110, 9771 (1999); A. Ballard, et al., J. Chem. Phys. 113. 5732 (2000)]. The reported results were obtained with the highest wavefunction-based level of theory ever applied to the C₆₀ system.
2008. "Prediction of the Vibrational Frequencies of UO₂²+ at the CCSD(T) Level." Journal of Physical Chemistry A 112(17):4095-4099. doi:10.1021/jp710334b Abstract Electronic structure calculations at the coupled cluster (CCSD(T)) and density functional theory levels with various relative effective core potentials and basis sets have been used to predict the isolated uranyl ion frequencies. The effects of anharmonicity and spin-orbit corrections on the harmonic frequencies have been calculated. The anharmonic effects are larger than the spin orbit corrections and both are small. The anharmonic effects decrease the frequencies and the spin orbit corrections increase the stretches and decrease the bend. Overall, these corrections decrease the harmonic asymmetric stretch frequency by 6 cm-¹, the symmetric stretch by 3 cm-1 and the bend by 3 cm-¹. The splitting between the asymmetric and symmetric stretch is predicted to be 86 cm-¹, which is consistent with experimental trends for substituted uranyls in solution and in the solid state.
2008. "Coupled-Cluster Dynamic Polarizabilities Including Triple Excitations." Journal of Chemical Physics 128:224102-1 - 224102-11. doi:10.1063/1.2929840 Abstract Dynamic polarizabilities for open- and closed-shell molecules were obtained using coupled-cluster (CC) linear response theory with full treatment of singles, doubles and triples (CCSDT-LR) with large basis sets utilizing the NWChem software suite. Using four approximate CC methods in conjunction with augmented cc-pVNZ basis sets, we are able to evaluate the convergence in both many-electron and one-electron spaces. For systems with primarily dynamic correlation, the results for CC3 and CCSDT are almost indistinguishable. For systems with more static correlation, the PS(T) approximation [J. Chem. Phs. 127, 164105 (2007) performs better that CC3. Additionally, the PS(T) approach separates the triples contribution to the poles of the response function from the triples amplitudes themselves, and demonstrates that the latter are less important than originally thought Lastly, our results show that the choice of reference (ROHF versus UHF) can have a significant impact on the accuracy of polarizabilities for open-shell systems.
2008. "Vibrational Spectroscopy of Anionic Nitrate Complexes of UO₂2+ and Eu³+ Isolated in the Gas Phase." Physical Chemistry Chemical Physics. PCCP 10(8):1192-1202. doi:10.1039/b715337f Abstract Wavelength-selective infrared multiple photon photo-dissociation (IRMPD) was used to generate infrared spectra of anionic nitrate complexes of UO₂²+ and Eu3+ in the mid-infrared region. A pattern of absorptions were observed for both species, including splitting of the antisymmetric O-N-O stretch into high and low frequency components with the magnitude of the splitting consistent with attachment of nitrate to a strong Lewis acid center. The frequencies measured for [UO2(NO3)3]- were within a few cm-1 of those measured in the condensed phase, the best agreement yet achieved for a comparison of IRMPD with condensed phase absorption spectra. In addition, experimentally-determined values were in good general agreement with those predicted by DFT calculations, especially for the antisymmetric UO₂ stretch. The spectrum from the [UO₂ (NO₃)₃]- was compared with that of [Eu(NO3)4]-, which showed that nitrate was bound more strongly to the Eu3+ metal center, consistent with its higher charge. The spectrum of a unique uranyl-oxo species having an elemental composition [UO9N₃]- was also acquired, for which calculations suggested a [UO₂(NO₃)₂(O)]- structure.
2008. "Infrared Spectroscopy of Dioxouranium (V) Complexes with Solvent Molecules: Effect of Reduction." Chemphyschem 9(9):1278-1285. doi:10.1002/cphc.200800034 Abstract UO2+-solvent complexes having the general formula [UO₂ (ROH)]+ (R = H, CH₃, C₂H₅, and n-C₃H₇) were formed using electrospray ionization and stored in a Fourier transform ion cyclotron resonance mass spectrometer, where they were isolated by mass-to-charge ratio, and then photofragmented using a free electron laser scanning through the 10 μm region of the infrared spectrum. Antisymmetric O=U=O stretching frequencies (ν3) were measured for all four complexes, which ranged from ~ 953 cm¯¹ for H₂O to ~ 944 cm¯¹ for n- PrOH, with the value for the EtOH-containing complex intermediate, systematically decreasing with increasing nucleophilicity of the solvent. The value for the MeOH-containing did not follow the trend, and had a measured ν3 value equal to that of the n-PrOH-containing complex. The ν3 frequency values for these U(V) complexes are comparable to those for the anionic [UO₂ (NO₃)₃]- complex, and lower than previously reported values for ligated uranyl (VI) dication complexes by 40 – 70 cm¯¹, and cationic uranyl (VI) ion-pair complexes by 10 – 40 cm¯¹. The lower frequency is attributed to weakening of the O=U=O bonds by repulsion related to reduction of the U metal center, which increases electron density in the antibonding π* orbitals of the uranyl moiety. Computational modelling of the ν3 frequencies of these species using PBE, B3LYP and LDA functionals showed good agreement with the IRMPD measurements. In general, expected trend in ν3 frequencies expected for the H₂O – MeOH – EtOH – n-PrOH series was produced by all three computational methods, however the three alcohols produced very similar values. The inverted order of MeOH and EtOH was not directly accounted for by the models, but is probably the result of overlapping C-H wagging modes that shift the apparent maxima of the O=U=O ν3 absorptions in the MeOH and EtOH complexes.
2008. "Infared Spectroscopy of Discrete Uranyl Anion Complexes." Journal of Physical Chemistry A 112(3):508-521. doi:10.1021/jp077309q Abstract The Free-Electron Laser for Infrared Experiments (FELIX) w 1 as used to study the wavelength-resolved multiple photon photodissociation of discrete, gas phase uranyl (UO2 2 2+) complexes containing a single anionic ligand (A), with or without ligated solvent molecules (S). The uranyl antisymmetric and symmetric stretching frequencies were measured for complexes with general formula [UO2A(S)n]+, where A was either hydroxide, methoxide, or acetate; S was water, ammonia, acetone, or acetonitrile; and n = 0-3. The values for the antisymmetric stretching frequency for uranyl ligated with only an anion ([UO2A]+) were as low or lower than measurements for [UO2]2+ ligated with as many as five strong neutral donor ligands, and are comparable to solution phase values. This result was surprising because initial DFT calculations predicted values that were 30–40 cm-1 higher, consistent with intuition but not with the data. Modification of the basis sets and use of alternative functionals improved computational accuracy for the methoxide and acetate complexes, but calculated values for the hydroxide were greater than the measurement regardless of the computational method used. Attachment of a neutral donor ligand S to [UO2A]+ produced [UO2AS]+, which produced only very modest changes to the uranyl antisymmetric stretch frequency, and did not universally shift the frequency to lower values. DFT calculations for [UO2AS]+ were in accord with trends in the data, and showed that attachment of the solvent was accommodated by weakening of the U-anion bond as well as the uranyl. When uranyl frequencies were compared for [UO2AS]+ species having different solvent neutrals, values decreased with increasing neutral nucleophilicity.
2008. "Fully Relativistic Calculations on the Potential Energy Surfaces of the Lowest 23 States of Molecular Chlorine." Journal of Chemical Physics 128(4):Art. No. 041101. doi:10.1063/1.2827457 Abstract The electronic structure and spectroscopic properties (Re, ωeϰe, βe, Te ) of the ground state and the 22 lowest excited states of chlorine molecule were studied within a four component relativistic framework using the MOLFDIR program package. The potential energy curves of all possible 23 covalent states were calculated using relativistic complete open shell configuration interaction (COSCI) approach. In addition, four component multi-reference configuration interaction with singles and doubles excitations (MRCISD) calculations were performed in order to infer the effects due to dynamical correlation in vertical excitations. The calculated properties are in good agreement with the available experimental data.
2008. "Two-Electron Three-Centered Bond in Side-On (η2) Uranyl(V) Superoxo Complexes." Journal of Physical Chemistry A 112(26):5777-5780. doi:10.1021/jp804202q Abstract Mononuclear dioxygen-metal compounds, such as FeO2 complexes with Schiff base and porphyrin ligands, play an essential role in chemistry ranging from oxyhemoglobin to cytochrome P-450 and cytochrome oxidase. It is well known that the superoxo complexes involved in these systems have end-on (η1) coordination geometries with O–O bond lengths of ~1.30 Å, Fe–O–O bond angles of ~135o, and vibrational frequencies of ~1150 cm-1,1 which reflects a formal change of the oxidation state from Mn+ to Mn+1. In addition, there are side-on (η2) peroxospecies, such as [Fe(porphyrin)(O2)]–, in which the O–O bond lengths is ~1.46 Å and vibrational frequencies of ~820 cm-1.1 These reflect a formal change of the oxidation state from Mn+ to Mn+2.
2007. "Direct Dynamics Trajectory Study of F− + CH3OOH Reactive Collisions Reveals a Major Non-IRC Reaction Path." Journal of the American Chemical Society 129(32):9976-9985. Abstract A direct dynamics simulation at the B3LYP/6-311+G(d,p) level of theory was used to study the reaction dynamics of F− + CH3OOH collisions. The results of the simulations are in excellent agreement with a previous experimental study (J. Am. Chem. Soc. 2002, 124, 3196). Two product channels, HF + CH2O + OH− and HF + CH3 OO−, are observed. The former dominates and occurs via an ECO2 mechanism in which F− attacks the CH3− group, abstracting a proton. Concertedly, a carbon-oxygen double bond is formed and OH− eliminated. Somewhat surprisingly this is not the reaction path, predicted by the intrinsic reaction coordinate (IRC), following F− attack of the CH3− group. The IRC leads to a deep potential energy minimum for the CH2(OH)2 · · · F− complex which dissociates to F− + CH2(OH)2. None of the direct dynamics trajectories followed the IRC, leading to this minimum and product channel. This channel has an exothermicity of 60 kcal/mol, much lower than the 27 kcal/mol exothermicity for the observed channel. Other channels not observed and which have lower exothermicities are F−+CO+H2+H2O (43 kcal/mol) and F− + CH2O + H2O (51 kcal/mol). Formation of a CH3OOH· · ·F− complex, with randomization of its internal energy, is important. This complex dissociates via the ECO2 mechanism forming HF + CH2O + OH−. Trajectories which form HF + CH3OO− are non-statistical events and, for the 4 ps direct dynamics simulation, are not mediated by a CH3OOH· · · F− complex. Dissociation of this complex to form HF + CH3OO− may occur on longer time scales.
2007. "Linear Response Coupled Cluster Singles and Doubles Approach with Modified Spectral Resolution of the Similarity Transformed Hamiltonian." Journal of Chemical Physics 127(16):164105 (9). doi:10.1063/1.2795708 Abstract This paper discusses practical scheme of correcting the linear response coupled cluster with singles and doubles (LR-CCSD) equations by shifting their poles, corresponding to the equation-of-motion CCSD (EOMCCSD) excitation energies, through adding the no-iterative corrections due to triples to the EOMCCSD excitation energies. A simple criterion is derived for the excited states to be corrected in the spectral resolution of similarity transformed Hamiltonian on the CCSD level. Benchmark calculations were performed to compare the accuracies of static and dynamic polarizabilities obtained in the way with the CC3 and CCSDT counterparts.
2007. "Dynamic Polarizabilities of Polyaromatic Hydrocarbons Using Coupled-Cluster Linear Response Theory." Journal of Chemical Physics 127:Art. No. 144105. doi:10.1063/1.2772853 Abstract Coupled-cluster theory with single and double excitations (CCSD) is applied to the calculation of optical properties of large polyaromatic hydrocarbons. Dipole polarizabilities are reported for benzene, pyrene, and the oligoacenes sequence n=2-6. Dynamic polarizabilities were calculated on polyacences as large asa pentacene for a single frequency and for benzene and pyrene at many frequencies. The basis set effect was studied for benzene using a variety of basis sets in the Pople and Dunning families up to aug-cc-pVQZ and the Sadlej p VTZ basis, which was used exclusively for the largest molecules. Optimized geometries were evaluated for some of the compounds using HF, B3LYP, PBE0, MP2 and CCSD to understand the role of bond-length-alteration (BLA). Finally, the polarizability results ere compared to four common density-functionals (B3LYP, BLYP, PBE0, PBE).
2007. "Calculations of Molecular Properties in Hybrid Coupled-Cluster and Molecular Mechanics Approach." Journal of Physical Chemistry A 111(25):5492-5498. doi:10.1021/jp070553x Abstract The ability to describe properties of molecular system in realistic environment is important for many applications. To address this issue we recently combined [M. Valiev, K. Kowalski, J. Chem. Phys. 2006, 125, 211101] classical molecular mechanics (MM) and ab initio coupled-cluster (CC) modules of NWChem. This paper reports the results of calculations of dipole moments and static polarizabilities for the C1₂0 system in the CC1₄ solution using the CCSD (CC with singles and doubles) linear response approach. We also discuss the application of the asymptotic extrapolation scheme (AES) [K. Kowalski, M. Valiev, J. Phys. Chem. A 2006, 110, 13106] in reducing the numerical cost of ab initio methods in the quantum region.
2007. "Mid-Infrared Vibrational Spectra of Discrete Acetone-Ligated Cerium Hydroxide Cations." Physical Chemistry Chemical Physics. PCCP 9(5):596-606. Abstract Cerium (III) hydroxy reactive sites are responsible for several important heterogeneous catalysis processes, and understanding the reaction chemistry of substrate molecules like CO, H2O, and CH3OH as they occur in heterogeneous media is a challenging task. We report here the first infrared spectra of model gas-phase cerium complexes and use the results as a benchmark to assist evaluation of the accuracy of ab initio calculations. Complexes containing [CeOH]2+ ligated by three- and four-acetone molecules were generated by electrospray ionization and characterized using wavelength-selective infrared multiple photon dissociation (IRMPD). The C=O stretching frequency for the [CeOH(acetone)4]2+ species appeared at 1650 cm-1 and was red-shifted by 90 cm-1 compared to unligated acetone. The magnitude of this shift for the carbonyl frequency was even greater for the [CeOH(acetone)3]2+ complex: the IRMPD peak consisted of two dissociation channels, an initial elimination of acetone at 1635 cm-1, and elimination of acetone accompanied by a serial charge separation producing [CeO(acetone)]+ at 1599 cm-1, with the overall frequency centered at 1616 cm-1. The increasing red shift observed as the number of acetone ligands decreases from four to three is consistent with transfer of more electron density per ligand in the less coordinated complexes. The lower frequency measured for the elimination/charge separation process is likely due to anharmonicity resulting from population of higher vibrational states. The C-C stretching frequency in the complexes is also influenced by coordination to the metal: it is blue-shifted compared to bare acetone, indicating a slight strengthening of the C-C bond in the complex, with the intensity of the absorption decreasing with decreasing ligation. Density functional theory (DFT) calculations using three different functionals (LDA, B3LYP, and PBE0) are used to predict the infrared spectra of the complexes. Calculated frequencies for the carbonyl stretch are within 40 cm-1 of the IRMPD of the three-acetone complex measured using the single acetone loss, and within 60 cm-1 of the measurement for the four-acetone complexes. The B3LYP and LDA functionals provided the best agreement with the measured spectra. The C-C stretching frequencies calculated using B3LYP are higher in energy than the measured values by ~ 30 cm-1, and reproduce the observed trend which shows that the C-C stretching frequency decreases with increasing ligation. Agreement between C-C frequency and calculation was not as good using the LDA functional, but still within 70 cm-1. The results provide an evaluation of changes in the acceptor properties of the metal center as ligands are added, and of the utility of DFT for modeling f-block coordination complexes.
2006. "Modeling Carbon Nanostructures with the Self-Consistent Charge Density-Functional Tight-Binding Method: Vibrational Spectra and Electronic Structure of C₂₈, C₆₀, and C₇₀." Journal of Chemical Physics 125(21):Art. No. 214706. Abstract The self-consistent charge density-functional tight-binding (SCC-DFTB) method is employed for studying various molecular properties of small fullerenes: C₂₈, C₆₀, and C₇₀. The computed optimized bond distances, vibrational infrared and Raman spectra, vibrational densities of states, and electronic densities of states are compared with experiment (where available) and density functional theory (DFT) calculations using various basis sets. The presented DFT benchmark calculations using the correlation-consistent polarized valence triple zeta (cc-pVTZ) basis set of Dunning are at present the most extensive calculations on harmonic frequencies of these species. Possible limitations of the SCC-DFTB method for the prediction of molecular vibrational and optical properties are discussed. The presented results suggest that SCC-DFTB is a computationally feasible and reliable method for predicting vibrational and electronic properties of such carbon nanostructures comparable in accuracy with small to medium size basis set DFT calculations at the computational cost of standard semiempirical methods.
2006. "Noniterative Inclusion of the Triply and Quadruply Excited Clusters: The Locally Renormalized Perspective." Journal of Molecular Structure - Theochem 768(1-3):45-52. Abstract Noniterative inclusion of the higher=order clusters has been a subject of intensive studies aimed at developing a well balanced description of individual many-body contributions for entire ground-state potential energy surfaces. In traditional approaches, the connected quadruples are estimated directly based on perturbative arguments, which leads to excellent agreement with full CI results near the equilibrium geometry and increasingly worse energies for larger internuclear stretches. As a possible improvement to this situation, two techniques are considered as especially promising: perturbative approaches based on the similarity transformed Hamiltonians and renormalization schemes both in global and local formulation. Following the latter strategy we adopted the recently introduced Numerator-Denominator Connected expansion (NDC) [ K. Kowalski, P. Piecuch, J. Chem. Phys. 122 (2005) [074107] as an effective tool for designing new forms of noniterative corrections accounting for the joint effect of triples and quadruples. The performance of the ensuing locally renormalized CCSD(TQ) approaches (LR-CCSD(TQ) is illustrated on several examples that require either going beyond the triples approximation or describing very subtle effects encountered in Van der Waals complexes. Comparisons with other noniterative approaches are also made and some issues regarding the size-extensivity of the locally renormalized methods are addressed.
2006. "Vibrational Spectroscopy of Mass-Selected [UO₂(ligand)n]²⁺ Complexes in the Gas Phase: Comparison with Theory." Journal of the American Chemical Society 128(14):4802-4813. doi:10.1021/ja058106n Abstract The gas-phase infrared spectra of discrete uranyl ([UO₂]²⁺) complexes ligated with acetone and/or acetonitrile were used to evaluate systematic trends of ligation on the position of the O=U=O stretch, and to enable rigorous comparison with the results of computational studies. Ionic uranyl complexes isolated in a Fourier transform ion cyclotron resonance mass spectrometer were fragmented via infrared multiphoton dissociation using a free electron laser scanned over the mid-IR wavelengths. The asymmetric O=U=O stretching frequency was measured at 1017 cm⁻¹ for [UO₂(CH₃COCH₃)₂]²⁺ and was systematically red shifted to 1000 and 988 cm⁻¹ by the addition of a third and fourth acetone ligand, respectively, which was consistent with increased donation of electron density to the uranium center in complexes with higher coordination number. The values generated computationally using LDA, B3LYP, and ZORA-PW91 were in good agreement with experimental measurements. In contrast to the uranyl frequency shifts, the carbonyl frequencies of the acetone ligands were progressively blue shifted as the number of ligands increased from 2 to 4, and approached that of free acetone. This observation was consistent with the formation of weaker noncovalent bonds between uranium and the carbonyl oxygen as the extent of ligation increases. Similar trends were observed for [UO₂(CH₃CN)n]²⁺ complexes, although the magnitude of the red shift in the uranyl frequency upon addition of more acetonitrile ligands was smaller than for acetone, consistent with the more modest nucleophilic nature of acetonitrile. This conclusion was confirmed by the uranyl stretching frequencies measured for mixed acetone/acetonitrile complexes, which showed that substitution of one acetone for one acetonitrile produced a modest red shift of 3 to 6 cm⁻¹.
2005. "Thermodynamic Properties of the C5, C6, and C8 n-Alkanes from Ab Initio Electronic Structure Theory." Journal of Physical Chemistry A 109:6934-6938. Abstract The heats of formation for the n-alkanes CnHn+2 for n=5, 6, 8 have been calculated using ab initio molecular orbital theory. Coupled cluster calculations with perturbative triples (CCSD(T)) were employed for the total valence electronic energies. Correlation consistent basis sets were used, up through the augmented quadruple zeta, to extrapolate to the complete basis set limit. Geometries were optimized at the B3LYP/TZVP and MP2/aug-cc-pVTZ levels. The MP2 geometries were used in the CCSD(T) calculations. Frequencies were determined at the density functional level (B3LYP/TZVP) and scaled zero-point energies were calculated from the B3LYP frequencies. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. The core-valence corrections are not small, (~1.1 kcal/mol per carbon unit) and cannot be neglected for chemical accuracy. The calculated ΔHf298 values are -35.0, -40.2, and -50.2 kcal/mol for C5H12, C6H14, and C8H18 respectively in excellent agreement with the respective experimental values of -35.11 ± 0.19, -39.89 ± 0.19, and -49.90 ± 0.31 kcal/mol. Isodesmic reaction energies are presented for some simple reactions involving C8H18 and are shown not to be strongly method dependent.
2005. "High-level ab-initio calculations for the four low-lying families of minima of (H2O)20: II. Spectroscopic signatures of the dodecahedron, fused cubes, face-sharing pentagonal prisms, and edge-sharing pentagonal prisms hydrogen bonding networks ." Journal of Chemical Physics 122(13):134304. Abstract We report the first harmonic vibrational spectra for each of the lowest lying isomers within the four major families of minima of (H2O)20, namely the dodecahedron, fused cubes, face-sharing pentagonal prisms and edge-sharing pentagonal prisms. These were obtained at the second-order Møller-Plesset perturbation level of theory (MP2) with the augmented correlation consistent basis set of double zeta quality (aug-cc-pVDZ) at the corresponding minimum energy geometries. The computed infrared (IR) spectra are the first ones obtained from first principles for these clusters. They were found to contain spectral features, which can be directly mapped onto the distinctive spectroscopic signatures of their constituent tetramer, pentamer and octamer fragments. The dodecahedron spectra show the richest structure in the OH stretching region and are associated with the most red-shifted OH vibrations with respect to the monomer. The lowest lying face-sharing pentagonal prism isomer displays intense IR active vibrations that are red-shifted by ~600 cm-1 with respect to the water monomer. The zero-point energy corrected MP2/CBS (complete basis set) limit binding energies (D0) for the four isomers are –163.1 kcal/mol (face-sharing pentagonal prism), -160.1 kcal/mol (edgesharing pentagonal prism), -157.5 kcal/mol (fused cubes) and –148.1 kcal/mol (dodecahedron).
2005. "Methyl Cation Affinities of Rare Gases and Nitrogen and the Heat of Formation of Diazomethane." Journal of Physical Chemistry A 109(18):4073-4080. doi:10.1021/jp044561e Abstract The methyl cation affinities of the rare gases have been calculated at 0 K and 298 K by using coupled cluster theory including noniterative, quasi-perturbative triple excitations level with the new correlation-consistent basis sets for Xe up through aug-cc-pV5Z in some cases. In order to achieve near chemical accuracy (± 1 kcal/mol) in the thermodynamic properties, we add to the estimated complete basis set valence binding energies based on frozen core coupled cluster theory energies two corrections: (1) a core/valence correction and (2) a scalar relativistic correction. Vibrational zero point energies were computed at the coupled cluster level of theory at the CCSD(T)/aug-cc-pVDZ level. The calculated rare gas methyl cation affinities (MCA) at 298 K are: MCA(He) = 1.7, MCA(Ne) = 2.5, MCA(Ar) = 16.9, MCA(Kr) = 25.5, and MCA(Xe) = 36.6 kcal/mol. Because of the importance of the MCA(N₂) in the experimental measurements of the MCA scale, we calculated a number of quantities associated with CH₃N₂⁺ and CH₂N₂. The calculated values for diazomethane at 298 K are: ΔHf(CH₂N₂) = 65.3, PA(CH₂N₂) = 211.9, and MCA(N₂) = 43.2 kcal/mol.
2005. "Heats of Formation of Xenon Fluorides and the Fluxionality of XeF₆ from High Level Electronic Structure Calculations." Journal of the American Chemical Society 127(24):8627-8634. doi:10.1021/ja0423116 Abstract Atomization energies at 0ºK and heats of formation at 0ºK were obtained for XeF⁺, XeF⁻, XeF₂, XeF₄, XeF₅⁻, and XeF₆ from coupled cluster theory including noniterative, quasi-perturbative triple excitations (CCSD(T)) calculations with new correlation-consistent basis sets for Xe up through aug-cc-pV5Z in some cases. In order to achieve near chemical accuracy (± 1 kcal/mol) in the thermodynamic properties, up to four corrections were added to the estimated complete basis set binding energies based on frozen core coupled cluster theory energies: (1) a correction for core-valence effects, (2) a correction for scalar relativistic effects, (3) a correction for first order atomic spin-orbit effects, and (4), in some cases, a second order spin-orbit correction. Vibrational zero point energies were computed at the coupled cluster level of theory. The structure of XeF6 is difficult to obtain. The C₃v and Oh structures for XeF₆ are essentially degenerate with the Oh structure below the C₃v structure by 0.19 kcal/mol at the CCSD(T)/CBS level based on an approximate geometry for the C₃v structure. The two structures are probably isoenergetic with the C₃v slightly lower. The calculated heats of formation are: ΔHf0 (XeF⁺) = 255.8, ΔHf0(XeF⁻) = -66.3, ΔHf0(XeF2) = -23.3, ΔHf0(XeF4) = -42.5, ΔHf0(XeF5⁻) = -160.6, and ΔHf0(XeF6) = -55.9 kcal/mol. The calculated heats of formation for the neutral XeFn fluorides are higher (less stable) than the experimental values from equilibrium measurements by 2.0, 7.7, and 12.2 kcal/mol for n =2, 4, and 6 respectively. The calculated values are even higher as compared to the heats of formation from photoionization measurements. The calculated results suggest that the experimental heats of formation need to be revised. The fluxional nature of the stereoactive lone pair in XeF6 is noted.
2005. "Complexation of the Carbonate, Nitrate, and Acetate Anions with the Uranyl Dication: Density Functional Studies with Relativistic Effective Core Potentials." Journal of Physical Chemistry A 109(50):11568-11577. doi:10.1021/jp0541462 Abstract The structures and vibrational frequencies of uranyl carbonates, [UO₂(CO₃)n](²- ²ⁿ) and [(UO₂)₃(CO₃)₆]⁶⁻, uranyl nitrates, [UO₂ (NO₃)n](²-ⁿ), and uranyl acetates, [UO₂(CH₃COO)n](²-ⁿ) (n = 1,2,3) have been calculated by using local density functional theory (LDFT). Only bidentate ligand coordination modes to the uranyl dication have been modeled. The calculated structures and frequencies are compared to available experimental data, including IR, Raman, X-ray diffraction, and EXAFS solution and crystal structure data. The energetics of ligand binding have been calculated using the B3LYP hybrid functional. In general, the structural and vibrational results at the LDFT level are in good agreement with experimental results and provide realistic pictures of solution phase and solid state behavior. For the [UO₂ (CO₃)₃]⁶⁻ anion, calculations suggest that complexity in the CO₃²⁻ stretching signature upon complexation is due to the formation of C=O and C-O domains, the latter of which can split by as much as 300 cm⁻¹. Assessment of the binding energies indicate that the [UO₂ (CO₃)₂]²⁻ anion is more stable than the [UO₂(CO₃)₃]⁴⁻ anion due to the accumulation of excess charge, whereas the tri-ligand species are the most stable in the nitrate and acetate anions.
2005. "Characterization of the molecular iodine electronic wave functions and potential energy curves through hyperfine interactions in the B0+_u(3Pi_u) state." Journal of the Optical Society of America. B, Optical Physics 22(5):951-961. Abstract We present a high-resolution numerical analysis on the six electronic states that are coupled to the B state via hyperfine interactions in molecular iodine. The four hyperfine parameters, CB, δB, dB, and eqQB, are calculated using the available potential energy curves and the wave functions constructed from the separated-atom basis set. In the calculation, we are able to obtain a maximum separation of the contributions from all the six electronic states and compare each individual term with the high-precision spectroscopic data, providing an independent verification of the relevant electronic structure.
2004. "Third-order Douglas-Kroll Relativistic Coupled-Cluster Theory through Connected Single, Double, Triple, and Quadruple Substitutions: Applications to Diatomic and Triatomic Hydrides." Journal of Chemical Physics 120(7):3297-3310. Abstract Coupled-cluster methods including through and up to the connected single, double, triple, and quadruple substitutions (CCSD, CCSDT, and CCSDTQ) have been automatically derived and implemented for sequential and parallel executions for use in conjunction with a one-component third-order Douglas-Kroll (DK3) approximation for relativistic corrections. A combination of the converging electron-correlation methods, the accurate relativistic reference wave functions, and the use of systematic basis sets tailored to the relativistic approximation has been shown to predict the experimental singlet-triplet separations within 0.02 eV (0.5 kcal/mol) for five triatomic hydrides (CH2, NH2+, SiH2, PH2+, and AsH2+), the experimental bond lengths within 0.002 angstroms, rotational constants within 0.02 cm-1, vibration-rotation constants within 0.01 cm-1, centrifugal distortion constants within 2 %, harmonic vibration frequencies within 9 cm-1 (0.4 %), anharmonic vibrational constants within 2 cm-1, and dissociation energies within 0.03 eV (0.8 kcal/mol) for twenty diatomic hydrides (BH, CH, NH, OH, FH, AlH, SiH, PH, SH, ClH, GaH, GeH, AsH, SeH, BrH, InH, SnH, SbH, TeH, and IH) containing main-group elements across the second through fifth periods of the periodic table. In these calculations, spin-orbit effects on dissociation energies, which were assumed to be additive, were estimated from the measured spin-orbit coupling constants of atoms and diatomic molecules, and an electronic energy in the complete-basis-set, complete-electron-correlation limit has been extrapolated by the formula which was in turn based on the exponential-Gaussian extrapolation formula of the basis set dependence.
2004. "Temperature and Isotope Substitute Effects on the Structure and NMR Properties of the Pertechnetate Ion in Water." Journal of the American Chemical Society 126(37):11583-11588. Abstract The uniquely well-resolved 99Tc NMR spectrum of the pertechnetate ion in liquid water poses a stringent test of the accuracy of ab initio calculations. The displacement of the 99Tc chemical shift as a function of temperature has been measured over the range 10-45 °C for the three isotopomers Tc(16O)4ˉ, Tc(16O)3(18O)ˉ, and Tc(16O)3(17O)ˉ at natural oxygen isotope abundance levels, and in addition the temperature dependence of the Tc-O scalar coupling was determined for the Tc(16O)3(17O)ˉ isotopomer. Values for these parameters were computed for three different theoretical models, which include both an unsolvated ion approximation and treatments of the solvated ion based on the COSMO approach. According to these analyses, the temperature- and isotope-dependent shifts in energy observed in the 99Tc NMR spectra can be quantitatively explained by changes in the average Tc-O bond length of the order of 1 x 10ˉ4 Å, which are induced by a redistribution of vibrational state populations and solvent-ion interactions. Vibrational energies and Tc-O bond lengths derived from these models also compare favorably with previous experimental determinations.
2003. "Intra Atomic Many-Body Effects in P-shell Photoelectron Spectra of Cr3+ Ions." Physical Review. B, Condensed Matter and Materials Physics 68(12):125106. Abstract A strict ab initio model of the many body effects for the free Cr3+ ion is developed in order to provide a new benchmark for intra-atomic effects in the XP spectra of Cr3+ compounds. The model contains no empirical fits or assumptions and incorporates all inter and intra shell couplings and recouplings, relativistic orbitals, spin-orbital coupling , the core hole, and Boltzmann weighted averages for initial state spin-orbit split levels. Synthetic Cr2p and 3p XP spectra are compared to an experimental spectrum of a-Cr2O3. The theory for the free Cr ion accounts for important features of the experimental Cr2p spectrum, including the spin-orbital splitting energy and the Cr2p3/2 doublet. In contrast, the theory only accounts for the grossest features of the experimental Cr3p spectrum. Comparing experiment with theory highlights the importance of both intra- and inter atomic effects on the XP spectra of Cr3+ compounds.
2003. "Performance of Coupled Cluster Theory in Thermochemical Calculations of Small Halogenated Compounds." Journal of Chemical Physics 118(8):3510-3522. Abstract Atomization energies at 0 K and heats of formation at 298 K were obtained for a collection of small halogenated molecules from coupled cluster theory including noniterative, quasi-perturbative triple excitations calculations with large basis sets (up through augmented septuple zeta quality in some cases). In order to achieve near chemical accuracy (1 kcal/mol) in the thermodynamic properties, we adopted a composite theoretical approach which incorporated estimated complete basis set binding energies based on frozen core coupled cluster theory energies and (up to) five corrections: (1) a core/valence correction; (2) a Douglas-Kroll-Hess scalar relativistic correction; (3) a first order atomic spin-orbit correction; (4) a second order spin-orbit correction for heavy elements and 5) an approximate correction to account for the remaining correlation energy. The last of these corrections is based on a recently proposed approximation to full configuration interaction via a continued fraction approximant for coupled cluster theory (CCSD(T)-cf). Failure to consider corrections (1) to (4) can introduce errors significantly in excess of the target accuracy of 1 kcal/mol. Although some cancellation of error may occur if one or more of these corrections is omitted, such a situation is by no means universal and cannot be relied upon for high accuracy. The accuracy of the Douglas-Kroll-Hess approach was calibrated against both new and previously published 4-component Dirac Coulomb results at the coupled cluster level of theory. In addition, vibrational zero point energies were computed at the coupled cluster level of theory for those polyatomic systems lacking an experimental anharmonic value.
2002. "Heats of formation of CBr, CHBr and CBr2 from ab initio quantum chemistry." Journal of Physical Chemistry A 106:4725-4728. Abstract High level ab initio electronic structure theory has been used to calculate the heats of formation of CBr, CHBr, and CBr2. The calculations were done at the CCSD(T) level with correlation-consistent basis sets up through augmented quintuple-z and extrapolated to the complete basis set limit. Additional corrections for core/valence correlation, relativistic effects both scalar and spin-orbit, and zero point energies have been included. The heat of formation at 0 K of CBr is 119.92 + 0.5 kcal/mol, of CHBr is 92.34 + 0.7 kcal/mol and that of CBr2 is 86.92 + 0.7 kcal/mol.
2002. "Fully Relativistic Correlated Benchmark Results for Uranyl and a Critical Look at Relativistic Effective Core Potentials for Uranium." Theoretical Chemistry Accounts 107(5):318. Abstract A complete set of fully-relativistic benchmark results for the bond lengths and vibrational frequencies of uranyl at various levels of correlation treatment are presented. It is shown that the relativistic and correlation affects are of the same magnitude and should be treated on an equal footing. Results of uranyl calculations using various relativistic effective core potentials (RECP) are presented. Appropriate choices for RECP's for use in density functional theory (DFT) calculations in the local density approximation (LDA) and with the gradient-corrected B3LYP exchange-correlation functional are discussed. The conclusion is reached that small-core RECP's need to be used and that the best results as compared to the benchmark values are obtained by using a DFT functional that includes some fraction of Hartree-Fock Exchange.
2001. "Parallel Douglas-Kroll Energy and Gradients in NWChem. Estimating Scalar Relativistic Effects Using Douglas-Kroll Contracted Basis Sets." Journal of Chemical Physics 114(1):48-53. Abstract A parallel implementation of the spin-free one-electron Douglas-Kroll(-Hess) Hamiltonian (DKH) in NWChem is discussed. An efficient and accurate method to calculate DKH gradients is introduced. It is shown that the use of standard (non-relativistic) contracted basis set can produce erroneous results for elements beyond the first row elements. The generation of DKH contracted cc-pVXZ (X = D, T, Q, 5) basis sets for H, He, B - Ne, Al - Ar, and Ga - Br will be discussed.
2001. "Comment on "Atomic Many-Body Effects for the p-Shell Photoelectron Spectra of Transition Metals" - Reply ." Physical Review Letters 86(16):3693. Abstract The comment by Taguchi et al. on our Letter about the importance of atomic contributions to the 2p and 3p x-ray photoelectron spectra (XPS) of ionic transition metal materials claims that we have "made serious misconceptions on" their calculations. They also criticize our work because we have not used empirical parameters to improve our agreement with experiment. We disagree with their criticisms.
2000. "Parallelization of Four-Component Calculations. I. Integral Generation, SCF, and Four-Index Transformation in the Dirac-Fock Package MOLFDIR." Journal of Computational Chemistry 21(13):1176-1186. Abstract The treatment of relativity and electron correlation on an equal footing is essential for the computation of systems containing heavy elements. Correlation treatments that are based on four-component Dirac-Hartree-Fock calculations presently provide the most accurate, albeit costly, way of taking relativity into account. The requirement of having two expansion basis sets for the molecular wave function puts a high demand on computer resources. The treatment of larger systems is thereby often prohibited by the very large run times and files that arise in a conventional Dirac-Hartree-Fock approach. A possible solution for this bottleneck is a parallel approach that not only reduces the turnaround time but also spreads out the large files over a number of local disks. Here, we present a distributed-memory parallelization of the program package MOLFDIR for the integral generation, Dirac-Hartree-Fock and four-index MS transformation steps. This implementation scales best for large AO spaces and moderately sized active spaces.
2000. "Atomic Many-Body Effects for the p-Shell Photoelectron Spectra of Transition Metals." Physical Review Letters 84(10):2259-2262. Abstract Ab initio theoretical results for the 2p- and 3p-hole states of an Mn²⁺ ion are reported in order to determine the importance of atomic contributions to the photoelectron spectra of bulk MnO. A combined treatment of relativity and electron correlation reveals important physical effects that have been neglected in virtually all previous work. The many-body and relativistic effects included in the atomic model are able, without any ad hoc empirical parameters, to explain most of the features of the MnO photoelectron spectra. In particular, it is not necessary to invoke charge transfer to explain the complex p-level spectra.
1999. "Structures and Binding Enthalpies of M+(H20)(n) Clusters, M = Cu, Ag, Au." Journal of Chemical Physics 110(3):1475-1491. Abstract Structures and incremental binding enthalpies were determined for the M+(H2O)n ionic clusters, M=Cu, Ag, Au; n=1–4 (5 for Cu) using correlated ab initio electronic structure methods. The effects of basis set expansion and high-level correlation recovery were found to be significant, in contrast to alkali and alkaline earth cation/water complexes, where correlation of the d electrons is unimportant. The use of a systematic sequence of one-particle basis sets permitted binding enthalpies in the complete basis set limit to be estimated. Overall, the best theoretical binding enthalpies compared favorably with the available experimental data for copper and silver. No experimental data is available for gold/water clusters.