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Dynamical properties of liquid water were studied using Car-Parrinello [Phys. Rev. Lett. 55, 2471 (1985)] ab initio molecular dynamics (AIMD) simulations within the Kohn-Sham (KS) density functional theory employing the Becke-Lee-Yang-Parr exchange-correlation functional for the electronic structure. The KS orbitals were expanded in a discrete variable representation basis set, wherein the complete basis set limit can be easily reached and which, therefore, provides complete convergence of ionic forces. In order to minimize possible nonergodic behavior of the simulated water system in a constant energy (NVE) ensemble, a long equilibration run (30 ps) preceded a 60 ps long production run. The temperature drift during the entire 60 ps trajectory was found to be minimal. The diffusion coefficient [0.055 Å2/ps] obtained from the present work for 32 D2O molecules is a factor of 4 smaller than the most up to date experimental value, but significantly larger than those of other recent AIMD studies. Adjusting the experimental result so as to match the finite-sized system used in the present study brings the comparison between theory and experiment to within a factor of 3. More importantly, the system is not observed to become “glassy” as has been reported in previous AIMD studies. The computed infrared spectrum is in good agreement with experimental data, especially in the low frequency regime where the translational and librational motions of water are manifested. The long simulation length also made it possible to perform detailed studies of hydrogen bond dynamics. The relaxation dynamics of hydrogen bonds observed in the present AIMD simulation is slower than those of popular force fields, such as the TIP4P potential, but comparable to that of the TIP5P potential. [ABSTRACT FROM AUTHOR]

The application of a modified Simon-Glatzel-type relation [Z. Anorg. Allg. Chem. 178, 309 (1929)] for the pressure evolution of the glass temperature is presented, namely, Tg(P)=Tg0[1+ΔP/(π+Pg0)]1/bexp[-(ΔP/c)], where (Tg0,Pg0) are the reference temperature and pressure, ΔP=P-Pg0, -π is the negative pressure asymptote, b is the power exponent, and c is the damping pressure coefficient. The discussion is based on the experimental Tg(P) data for magmatic silicate melt albite, polymeric liquid crystal P8, and glycerol. The latter data are taken from Cook et al. [J. Chem. Phys. 100, 5178 (1994)] and from the authors’ dielectric relaxation time (τ(P)) measurements, which employs the novel pressure counterpart of the Vogel-Fulcher-Tammann equation: τ(P)=τ0P exp[DPΔP/(P0-P)], where ΔP=P-PSL (PSL is the stability limit hidden under negative pressure), P0 is the estimation of the ideal glass pressure, and DP is the isothermal fragility strength coefficient. Results obtained suggest the hypothetical maximum of the Tg(P) curve, which can be estimated due to the application of the supporting derivative-based analysis. A hypothetical common description of glass formers characterized by dTg/dP>0 and dTg/dP<0 coefficients is suggested. Finally, the hypothetical link between molecular and colloidal glass formers is recalled. [ABSTRACT FROM AUTHOR]

Simulation (molecular dynamics and Car-Parrinello [Phys. Rev. Lett. 55, 2471 (1985)]) and diffraction (x-ray and neutron) studies on nitromethane are compared aiming at the determination of the liquid structure. Beyond that, the capabilities of the methods to describe liquid structure are discussed. For the studied liquid, the diffraction methods are performing very well in the determination of intramolecular structure, but they do not give detailed structural information on the intermolecular structure. The good agreement between the diffraction experiments and the results of molecular dynamics simulations justifies the use of simulations for the more detailed description of the liquid structure using partial radial distribution functions and orientational correlation functions. Liquid nitromethane is described as a molecular liquid without strong intermolecular interactions such as hydrogen bonding, but with detectable orientational correlations resulting in preferential antiparallel order of the neighboring molecules. [ABSTRACT FROM AUTHOR]

The authors present a study of the nonequilibrium statistical properties of a one dimensional hard-rod fluid dissipating energy via inelastic collisions and subject to the action of a Gaussian heat bath, simulating an external driving mechanism. They show that the description of the fluid based on the one-particle phase-space reduced distribution function, in principle necessary because of the presence of velocity dependent collisional dissipation, can be contracted to a simpler description in configurational space. Indeed, by means of a multiple-time-scale method the authors derive a self-consistent governing equation for the particle density distribution function. This equation is similar to the dynamic density functional equation employed in the study of colloids, but contains additional terms taking into account the inelastic nature of the fluid. Such terms cannot be derived from a Liapunov generating functional and contribute not only to the relaxational properties, but also to the nonequilibrium steady state properties. A validation of the theory against molecular dynamics simulations is presented in a series of cases, and good agreement is found. [ABSTRACT FROM AUTHOR]

The properties of aqueous sulfuric acid have been studied employing density functional theory–based molecular dynamics simulations in conjunction with norm-conserving pseudopotentials. The simulations were carried out for two different concentrations whose molar concentrations were fixed at 0.84 and 10.2 mol/l. The structural features of aqueous sulfuric acid solutions show a strong dependency on the concentration. The Grötthuss-type proton transfer mechanism is not effectively operative at the higher concentration because of the broken hydrogen bond network of water induced by ions generated by the dissociation of sulfuric acid. In addition, to evaluate electrical properties, we carried out a simulation that takes an electric field into account. Results are compared with those of the simulation undertaken with no external electric field. [ABSTRACT FROM AUTHOR]

The mechanical elongation of a finite gold nanowire has been studied by molecular dynamics simulations using different semiempirical potentials for transition metals. These potentials have been widely used to study the mechanical properties of finite metal clusters. Combining with density functional theory calculations along several atomic-configuration trajectories predicted by different semiempirical potentials, the authors conclude that the second-moment approximation of the tight-binding scheme (TB-SMA) potential is the most suitable one to describe the energetics of finite Au clusters. They find that for the selected geometries of Au wires studied in this work, the ductile elongation of Au nanowires along the [001] direction predicted by the TB-SMA potential is largely independent of temperature in the range of 0.01–298 K. The elongation leads to the formation of monatomic chains, as has been observed experimentally. The calculated force-versus-elongation curve is remarkably consistent with available experimental results. [ABSTRACT FROM AUTHOR]

By using the exact density functional theory, one demonstrates that the value of the local electronic softness of a molecular fragment is directly related to the polarization charge (Coulomb hole) induced by a test electron removed (or added) from (at) the fragment. Our finding generalizes to a chemical group a formal relation between these molecular descriptors recently obtained for an atom in a molecule using an approximate atomistic model [P. Senet and M. Yang, J. Chem. Sci. 117, 411 (2005)]. In addition, a practical ab initio computational scheme of the Coulomb hole and related local descriptors of reactivity of a molecular family having in common a similar fragment is presented. As a blind test, the method is applied to the lateral chains of the 20 isolated amino acids. One demonstrates that the local softness of the lateral chain is a quantitative measure of the similarity of the amino acids. It predicts the separation of amino acids in different biochemical groups (aliphatic, basic, acidic, sulfur contained, and aromatic). The present approach may find applications in quantitative structure activity relationship methodology. [ABSTRACT FROM AUTHOR]

The Gaussian-4 theory (G4 theory) for the calculation of energies of compounds containing first- (Li–F), second- (Na–Cl), and third-row main group (K, Ca, and Ga–Kr) atoms is presented. This theoretical procedure is the fourth in the Gaussian-n series of quantum chemical methods based on a sequence of single point energy calculations. The G4 theory modifies the Gaussian-3 (G3) theory in five ways. First, an extrapolation procedure is used to obtain the Hartree-Fock limit for inclusion in the total energy calculation. Second, the d-polarization sets are increased to 3d on the first-row atoms and to 4d on the second-row atoms, with reoptimization of the exponents for the latter. Third, the QCISD(T) method is replaced by the CCSD(T) method for the highest level of correlation treatment. Fourth, optimized geometries and zero-point energies are obtained with the B3LYP density functional. Fifth, two new higher level corrections are added to account for deficiencies in the energy calculations. The new method is assessed on the 454 experimental energies in the G3/05 test set [L. A. Curtiss, P. C. Redfern, and K. Raghavachari, J. Chem. Phys. 123, 124107 (2005)], and the average absolute deviation from experiment shows significant improvement from 1.13 kcal/mol (G3 theory) to 0.83 kcal/mol (G4 theory). The largest improvement is found for 79 nonhydrogen systems (2.10 kcal/mol for G3 versus 1.13 kcal/mol for G4). The contributions of the new features to this improvement are analyzed and the performance on different types of energies is discussed. [ABSTRACT FROM AUTHOR]

Event-driven molecular dynamics simulations are carried out on two rigid-body systems which differ in the symmetry of their molecular mass distributions. First, simulations of methane in which the molecules interact via discontinuous potentials are compared with simulations in which the molecules interact through standard continuous Lennard-Jones potentials. It is shown that under similar conditions of temperature and pressure, the rigid discontinuous molecular dynamics method reproduces the essential dynamical and structural features found in continuous-potential simulations at both gas and liquid densities. Moreover, the discontinuous molecular dynamics approach is demonstrated to be between 3 and 100 times more efficient than the standard molecular dynamics method depending on the specific conditions of the simulation. The rigid discontinuous molecular dynamics method is also applied to a discontinuous-potential model of a liquid composed of rigid benzene molecules, and equilibrium and dynamical properties are shown to be in qualitative agreement with more detailed continuous-potential models of benzene. The few qualitative differences in the angular dynamics of the two models are related to the relatively crude treatment of variations in the discontinuous repulsive interactions as one benzene molecule rotates by another. [ABSTRACT FROM AUTHOR]

As model compounds for nanosize carbon clusters, the phonon dispersion curves of polyacene are constructed based on density functional theory calculations for [n]oligoacenes (n=2-5, 10, and 15). Complete vibrational assignments are given for the observed Fourier-transform infrared and Raman spectra of [n]oligoacenes (n=2-5). Raman intensity distributions by the 1064-nm excitation are well reproduced by the polarizability-approximation calculations for naphthalene and anthracene, whereas several bands of naphthacene and pentacene at 1700-1100 cm-1 are calculated to be enhanced by the resonance Raman effect. It is found from vibronic calculations that the coupled ag modes between the Kekulé deformation and joint CC stretching give rise to the Raman enhancements of the Franck–Condon type, and that the b3g mode corresponding to the graphite G mode is enhanced by vibronic coupling between the 1La(1B1u) and 1Bb(1B2u) states. The phonon dispersion curves of polyacene provide a uniform foundation for understanding molecular vibrations of the oligoacenes in terms of the phase difference. The mode correlated with the defect-sensitive D mode of the bulk carbon networks is also found for the present one-dimensional system. [ABSTRACT FROM AUTHOR]

The authors use ab initio molecular dynamics and the density functional theory+U (DFT+U) method to compute the hydration environment of the manganese ion in manganese (II) and manganese (III) porphines (MnP) dispersed in liquid water. These are intended as simple models for more complex water soluble porphyrins, which have important physiological and electrochemical applications. The manganese ion in Mn(II)P exhibits significant out-of-porphine plane displacement and binds strongly to a single H2O molecule in liquid water. The Mn in Mn(III)P is on average coplanar with the porphine plane and forms a stable complex with two H2O molecules. The residence times of these water molecules exceed 15 ps. The DFT+U method correctly predicts that water displaces NO from Mn(III)P–NO, but yields an ambiguous spin state for the MnP(II)–NO complex. [ABSTRACT FROM AUTHOR]

We have carried out a series of molecular dynamics simulations to investigate the hydrogen bond and residence dynamics of X-–water (X=F, Cl, and I) and pairs in aqueous solutions at a temperature of 673 K. The calculations are done at six different water densities ranging from 1.0 to 0.15 g cm-3. The hydrogen bonds are defined by using a set of configurational criteria with respect to the anion(oxygen)–oxygen and anion(oxygen)–hydrogen distances and the anion(oxygen)–oxygen–hydrogen angle for an anion(water)–water pair. The F-–water hydrogen bonds are found to have a longer lifetime than all other hydrogen bonds considered in the present study. The lifetime of Cl-–water hydrogen bonds is shorter than that of F-–water hydrogen bonds but longer than the lifetime of water–water hydrogen bonds. The lifetimes of I-–water and water–water hydrogen bonds are found to be very similar. Generally, the lifetimes of both anion–water and water–water hydrogen bonds are found to be significantly shorter than those found under ambient conditions. In addition to hydrogen bond lifetimes, we have also calculated the residence times and the orientational relaxation times of water molecules in ion(water) hydration shells and have discussed the correlations of these dynamical quantities with the observed dynamics of anion(water)-water hydrogen bonds as functions of the ion size and density of the supercritical solutions. [ABSTRACT FROM AUTHOR]

Molecular density profiles and charge distributions determined by density functional theory (DFT) are used in conjunction with the continuum Navier-Stokes equations to compute electro-osmotic flows in nanoscale channels. The ion species of the electrolyte are represented as centrally charged hard spheres, and the solvent is treated as a dense fluid of neutral hard spheres having a uniform dielectric constant. The model explicitly accounts for Lennard-Jones interactions among fluid and wall molecules, hard sphere repulsions, and short range electrical interactions, as well as long range Coulombic interactions. Only the last of these interactions is included in classical Poisson-Boltzmann (PB) modeling of the electric field. Although the proposed DFT approach is quite general, the sample calculations presented here are limited to symmetric monovalent electrolytes. For a prescribed surface charge, this DFT model predicts larger counterion concentrations near charged channel walls, relative to classical PB modeling, and hence smaller concentrations in the channel center. This shifting of counterions toward the walls reduces the effective thickness of the Debye layer and reduces electro-osmotic velocities as compared to classical PB modeling. Zeta potentials and fluid speeds computed by the DFT model are as much as two or three times smaller than corresponding PB results. This disparity generally increases with increasing electrolyte concentration, increasing surface charge density and decreasing channel width. The DFT results are found to be comparable to those obtained by molecular dynamics simulation, but require considerably less computing time. [ABSTRACT FROM AUTHOR]

The behavior of a fluid inside a closed narrow slit between solid walls is examined on the basis of the density functional theory. It is shown that the constraint of constant number of molecules leads to interesting effects which are absent when the slit is open and in contact with a reservoir. If the slit walls are identical, the density profiles at low temperatures or at high average densities ρav of the fluid molecules in the slit have a sharp maximum in the middle of the slit, the value of the density at maximum being comparable to that of a liquid. The density of fluid at the walls is in this case comparable to the density of a vapor phase. At high temperatures or at low ρav the fluid density in the middle of the slit is of the same order of magnitude as at the walls. For nonidentical walls the density maximum is shifted towards the wall with a stronger wall-fluid interaction. The transition between the two types (with and without the sharp maximum) of density profiles with the change of temperature in the slit occurs in a narrow range of temperatures, this range being larger for narrower slits. The pressures which the fluid exerts on the walls as well as the forces per unit area arising due to stresses in the sidewalls of the system can decrease with increasing ρav. Such a behavior is not possible for homogeneous systems and can be explained by analyzing the fluid density at the walls when ρav increases. The normal and transversal components of the pressure tensor were calculated as functions of the distance from the wall using the equation of hydrostatic equilibrium and direct calculation of the forces between molecules, respectively. The normal component decreases with increasing distance near the wall in contrast to the normal component near the liquid-vapor interface reported previously in the literature. The behavior of the transverse component does not depend on the fluid-solid interaction and is comparable to that for a liquid-vapor interface. [ABSTRACT FROM AUTHOR]

In order to better understand the physical interactions that stabilize protein secondary structure, the neat liquid state of a peptidic fragment, N-methylacetamide (NMA), was studied using computer simulation. Three different descriptions of the molecular liquid were examined: an empirical force field treatment with classical nuclei, an empirical force field treatment with quantum mechanical nuclei, and an ab initio density functional theory (DFT) treatment. The DFT electronic structure was evaluated using the BLYP approximate functional and a plane wave basis set. The different physical effects probed by the three models, such as quantum dispersion, many-body polarization, and nontrivial charge distributions on the liquid properties, were compared. Much of the structural ordering in the liquid is characterized by hydrogen bonded chains of NMA molecules. Modest structural differences are present among the three models of liquid NMA. The average molecular dipole in the liquid under the ab initio treatment, however, is enhanced by 60% over the gas phase value. [ABSTRACT FROM AUTHOR]

Nuclear-spin relaxation is considered in a molecular system undergoing two types of dynamic processes: asymmetric-top small-step rotational diffusion and discrete multisite local jumps. The two processes are assumed to be uncorrelated. Time correlation functions for relevant rank-two interactions and corresponding spectral density functions are derived for a general relation between the characteristic rate constants. In addition, limiting cases of fast and slow local motions and of some specific jump conditions are also investigated. [ABSTRACT FROM AUTHOR]

We report an ab initio-based global potential energy surface for H+CH4 that describes the abstraction and exchange reactions. The PES, which is invariant with respect to any permutation of five H atoms, is a fit to 20 728 electronic energies calculated using the partially spin-restricted coupled-cluster method (RCCSD(T)) with a moderately large basis (aug-cc-pVTZ). A first set of quasiclassical trajectory calculations using this PES are reported for the H+CD4→HD+CD3 reaction at collision energies of 0.65 and 1.52 eV and are compared to experiment and recent direct dynamics calculations done with density functional theory. [ABSTRACT FROM AUTHOR]

We present a study of the structure in the interface between two immiscible liquids by density-functional theory and molecular-dynamics calculations. The liquids are modeled by Lennard-Jones potentials, which achieve immiscibility by supressing the attractive interaction between unlike particles. The density profiles of the liquids display oscillations only in a limited part of the simple liquid-phase diagram (ρ,T). When approaching the liquid-vapor coexistence, a significant depletion appears while the layering behavior of the density profile vanishes. By analogy with the liquid-vapor interface and the analysis of the adsorption this behavior is suggested to be strongly related to the drying transition. [ABSTRACT FROM AUTHOR]

The physicochemical properties of two molten salts, namely, KCl and NaCl, have been studied with a molecular-dynamics approach using a density-functional-based tight-binding (DFTB) model. The obtained results have been compared with a number of previously reported simulations, carried out on smaller systems and using classical force-field techniques. A good agreement has been found for both structural parameters and macroscopic properties, such as self-diffusion coefficients. Furthermore, our DFTB results are very close to the available experimental data. From a more general point of view, our results demonstrate the applicability of DFTB as an efficient tool in the modeling of melts. At the same time, the quality of the obtained results supports the use of this as a reliable alternative to the more expensive ab initio dynamics approaches, if accurate parameters are provided. [ABSTRACT FROM AUTHOR]

We extend our previous formulation of time-dependent four-component relativistic density-functional theory [J. Gao, W. Liu, B. Song, and C. Liu, J. Chem. Phys. 121, 6658 (2004)] by using a noncollinear form for the exchange-correlation kernel. The new formalism can deal with excited states involving moment (spin)-flipped configurations which are otherwise not accessible with ordinary exchange-correlation functionals. As a first application, the global potential-energy curves of 16 low-lying ωω-coupled electronic states of the AuH molecule have been investigated. The derived spectroscopic parameters, including the adiabatic and vertical excitation energies, equilibrium bond lengths, harmonic and anharmonic vibrational constants, fundamental frequencies, and dissociation energies, are grossly in good agreement with those of ab initio multireference second-order perturbation theory and the available experimental data. [ABSTRACT FROM AUTHOR]

The static and dynamical properties of heavy water have been studied at ambient conditions with extensive Car–Parrinello molecular-dynamics simulations in the canonical ensemble, with temperatures ranging between 325 and 400 K. Density-functional theory, paired with a modern exchange-correlation functional (Perdew–Burke–Ernzerhof), provides an excellent agreement for the structural properties and binding energy of the water monomer and dimer. On the other hand, the structural and dynamical properties of the bulk liquid show a clear enhancement of the local structure compared to experimental results; a distinctive transition to liquidlike diffusion occurs in the simulations only at the elevated temperature of 400 K. Extensive runs of up to 50 ps are needed to obtain well-converged thermal averages; the use of ultrasoft or norm-conserving pseudopotentials and the larger plane-wave sets associated with the latter choice had, as expected, only negligible effects on the final result. Finite-size effects in the liquid state are found to be mostly negligible for systems as small as 32 molecules per unit cell. [ABSTRACT FROM AUTHOR]

A (quasirelativistic) two-component density functional theory (DFT) approach to the computation of parity-violating energy differences between enantiomers is presented which is based on the zeroth-order regular approximation (ZORA). This approach is employed herein to compute parity-violating energy differences between several P and M conformations of dihydrogen dichalcogenides (H2X2 with X=O, S, Se, Te, Po), of which some compounds have recently been suggested as potential molecular candidates for the first experimental measurement of parity-violating effects in chiral molecules. The DFT ZORA results obtained in this work with “pure” density functionals are anticipated to deviate by well less than 1% from data that would be computed within related (relativistic) four-component Dirac–Kohn–Sham–Coulomb schemes. In our implementation of the ZORA slightly larger relative deviations are expected for hybrid functionals, depending on the amount of “exact” exchange. For B3LYP (20% exact exchange) differences are estimated to amount to at most 3% in hydrogen peroxide, 2% in disulfane, and 1% or less for the heavier homologs. Thus, the present two-component approach is expected to perform excellently when compared to four-component density functional schemes while being at the same time computationally more efficient. The ZORA approach will therefore be of particular interest for the prediction of parity-violating vibrational frequency shifts, for instance, in isotopomers of H2Se2 and H2Te2. [ABSTRACT FROM AUTHOR]

A methodology to efficiently conduct simultaneous dynamics of electrons and nuclei is presented. The approach involves quantum wave packet dynamics using an accurate banded, sparse and Toeplitz representation for the discrete free propagator, in conjunction with ab initio molecular dynamics treatment of the electronic and classical nuclear degree of freedom. The latter may be achieved either by using atom-centered density-matrix propagation or by using Born–Oppenheimer dynamics. The two components of the methodology, namely, quantum dynamics and ab initio molecular dynamics, are harnessed together using a time-dependent self-consistent field-like coupling procedure. The quantum wave packet dynamics is made computationally robust by using adaptive grids to achieve optimized sampling. One notable feature of the approach is that important quantum dynamical effects including zero-point effects, tunneling, as well as over-barrier reflections are treated accurately. The electronic degrees of freedom are simultaneously handled at accurate levels of density functional theory, including hybrid or gradient corrected approximations. Benchmark calculations are provided for proton transfer systems and the dynamics results are compared with exact calculations to determine the accuracy of the approach. [ABSTRACT FROM AUTHOR]

We use density functional theory based molecular-dynamics simulations to study the aqueous solvation of the fluoride anion. Our studies are focused on the first solvation shell and have resulted in detailed information on its structural and dynamical properties. The fluoride ion leads to the formation of a rigid solvation shell, qualitatively consistent with simulation and experimental studies, classifying fluoride as a “structure making” particle. However, quantitatively we find the solvation shell to be less structured and more mobile than predicted from empirical force-field simulation. The influence on the intramolecular electronical and structural properties of water is minimal, as observed for other halogens. We propose two distinct mechanisms for the exchange of bulk and first solvation shell water molecules. [ABSTRACT FROM AUTHOR]

The orientational dependence of the interaction between two C60 molecules is investigated using ab initio calculations. The binding energy, computed within density functional theory in the local density approximation, is substantially smaller than the one derived from the experimental heat of sublimation of fullerite, which calls into question the nature of inter-C60 bonding. According to our calculations, the experimentally observed orientation with a C60 presenting a hexagon-hexagon bond to a pentagonal face of the other C60 is not really favored. Some other configurations are very close in energy and in fact a pentagon facing a pentagon and a hexagon facing a hexagon-hexagon bond are found to be slightly more favorable situations. Our results are compared to previous ones obtained either with previous empirical intermolecular potentials or to existing ab initio studies of crystalline C60. In addition, the stacking of C60 in a crystal and in a decahedral (C60)7 cluster is discussed. [ABSTRACT FROM AUTHOR]

Approximations to the Breit-Pauli form of the spin-orbit coupling (SOC) operator are examined. The focus is on approximations that lead to an effective quasi-one-electron operator which leads to efficient property evaluations. In particular, the accurate spin-orbit mean-field (SOMF) method developed by Hess, Marian, Wahlgren, and Gropen is examined in detail. It is compared in detail with the “effective potential” spin-orbit operator commonly used in density functional theory (DFT) and which has been criticized for not including the spin-other orbit (SOO) contribution. Both operators contain identical one-electron and Coulomb terms since the SOO contribution to the Coulomb term vanishes exactly in the SOMF treatment. Since the DFT correlation functional only contributes negligibly to the SOC the only difference between the two operators is in the exchange part. In the SOMF approximation, the SOO part is equal to two times the spin-same orbit contribution. The DFT exchange contribution is of the wrong sign and numerically shown to be in error by a factor of 2–2.5 in magnitude. The simplest possible improvement in the DFT-SOC treatment [Veff(-2X)-SOC] is to multiply the exchange contribution to the Veff operator by -2. This is verified numerically in calculations of molecular g-tensors and one-electron SOC constants of atoms and ions. Four different ways of handling the computationally critical Coulomb part of the SOMF and Veff operators are discussed and implemented. The resolution of the identity approximation is virtually exact for the SOC with standard auxiliary basis sets which need to be slightly augmented by steep s functions for heavier elements. An almost as efficient seminumerical approximation is equally accurate. The effective nuclear charge model gives results within ∼10% (on average) of the SOMF treatment. The one-center approximation to the Coulomb and one-electron SOC terms leads to errors on the order of ∼5%. Small absolute errors are obtained for the one-center approximation to the exchange term which is consequently the method of choice [SOMF(1X)] for large molecules.© 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose approximations which go beyond the local-density approximation for the short-range exchange and correlation density functionals appearing in a multideterminantal extension of the Kohn–Sham scheme. A first approximation consists of defining locally the range of the interaction in the correlation functional. Another approximation, more conventional, is based on a gradient expansion of the short-range exchange-correlation functional. Finally, we also test a short-range generalized-gradient approximation by extending the Perdew–Burke–Ernzerhof exchange-correlation functional to short-range interactions. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The performance of density functional theory methods for the modeling of condensed aqueous systems is hard to predict and validation by ab initio molecular simulation of liquid water is absolutely necessary. In order to assess the reliability of these tests, the effect of temperature on the structure and dynamics of liquid water has been characterized with 16 simulations of 20 ps in the temperature range of 280–380 K. We find a pronounced influence of temperature on the pair correlation functions and on the diffusion constant including nonergodic behavior on the time scale of the simulation in the lower temperature range (which includes ambient temperature). These observations were taken into account in a consistent comparison of a series of density functionals (BLYP, PBE, TPSS, OLYP, HCTH120, HCTH407). All simulations were carried out using an ab initio molecular dynamics approach in which wave functions are represented using Gaussians and the density is expanded in an auxiliary basis of plane waves. Whereas the first three functionals show similar behavior, it is found that the latter three functionals yield more diffusive dynamics and less structure.© 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The UV absorption of aqueous Cu+ and Ag+ has been studied using Time Dependent Density Functional Theory (TDDFT) response techniques. The TDDFT electronic spectrum was computed from finite temperature dynamical trajectories in solution generated using the Density Functional Theory (DFT) based Ab Initio Molecular Dynamics (AIMD) method. The absorption of the two ions is shown to arise from similar excitation mechanisms, namely transitions from d orbitals localized on the metal center to a rather delocalized state originating from hybridization of the metal s orbital to the conduction band edge of the solvent. The ions differ in the way the spectral profile builds up as a consequence of solvent thermal motion. The Cu+ absorption is widely modulated, both in transition energies and intensities by fluctuations in the coordination environment which is characterized by the formation of strong coordination bonds to two water molecules in an approximately linear geometry. Though, on average, absorption intensities are typical of symmetry forbidden transitions of metal ions in the solid state, occasionally very short (<100 fs) bursts in intensity are observed, associated with anomalous Cu–H interactions. Absorption by the Ag+ complex is in comparison relatively stable in time, and can be interpreted in terms of the energy splitting of the metal 4d manifold in an average crystal field corresponding to a fourfold coordination in a distorted tetrahedral arrangement. Whereas the spectral profile of the Ag+ aqua ion is in good agreement with experiment, the overall position of the band is underestimated by 2 eV in the BLYP approximation to DFT. The discrepancy with experiment is reduced to 1.3 eV when a hybrid functional (PBE0) is used. The remaining inaccuracy of TDDFT in this situation is related to the delocalized character of the target state in d→s transitions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A series of 20 ps ab initio molecular dynamics simulations of water at ambient density and temperatures ranging from 300 to 450 K are presented. Car-Parrinello (CP) and Born-Oppenheimer (BO) molecular dynamics techniques are compared for systems containing 54 and 64 water molecules. At 300 K, an excellent agreement is found between radial distribution functions (RDFs) obtained with BO and CP dynamics, provided an appropriately small value of the fictitious mass parameter is used in the CP simulation. However, we find that the diffusion coefficients computed from CP dynamics are approximately two times larger than those obtained with BO simulations for T>400 K, where statistically meaningful comparisons can be made. Overall, both BO and CP dynamics at 300 K yield overstructured RDFs and slow diffusion as compared to experiment. In order to understand these discrepancies, the effect of proton quantum motion is investigated with the use of empirical interaction potentials. We find that proton quantum effects may have a larger impact than previously thought on structure and diffusion of the liquid. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new Kohn–Sham formalism is developed for studying the lowest molecular electronic states of given space and spin symmetry whose densities are represented by weighted sums of several reference configurations. Unlike standard spin-density functional theory, the new formalism uses total spin conserving spin-density operators and spin-invariant density matrices so that the method is fully spin-adapted and solves the so-called spin-symmetry dilemma. The formalism permits the use of an arbitrary set of reference (noninteracting) configurations with any number of open shells. It is shown that the requirement of degeneracy of the total noninteracting energies of the reference configurations (or configuration state functions) is equivalent to the stationary condition of the exact energy relative to the weights of the configurations (or configuration state functions). Consequently, at any molecular geometry, the weights can be determined by minimization of the energy, and, for given reference weights, the Kohn–Sham orbitals can be determined. From this viewpoint, the developed theory can be interpreted as an analog of the multiconfiguration self-consistent field approach within density functional theory. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Density functional theory is used to give an operational definition for the “free energy surface” on which Si adatoms move over a (100) silicon surface. Molecular dynamics simulations using the empirical Stillinger–Weber potential are used to numerically evaluate free energy surfaces near binding sites and along the important hopping paths. Transition state theory is used to deduce hopping rates at T≈500 and 1000 K from these free energy surfaces for the basic steps responsible for “surface diffusion.” The difference in the activation free energies at these temperatures and the T=0 barrier heights is emphasized.© 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Relaxation in supercooled liquids above their glass transition and below the onset temperature of “slow” dynamics involves the correlated motion of neighboring particles. This correlated motion results in the appearance of spatially heterogeneous dynamics or “dynamical heterogeneity.” Traditional two-point time-dependent density correlation functions, while providing information about the transient “caging” of particles on cooling, are unable to provide sufficiently detailed information about correlated motion and dynamical heterogeneity. Here, we study a four-point, time-dependent density correlation function g[sub 4](r,t) and corresponding “structure factor” S[sub 4](q,t) which measure the spatial correlations between the local liquid density at two points in space, each at two different times, and so are sensitive to dynamical heterogeneity. We study g[sub 4](r,t) and S[sub 4](q,t) via molecular dynamics simulations of a binary Lennard-Jones mixture approaching the mode coupling temperature from above. We find that the correlations between particles measured by g[sub 4](r,t) and S[sub 4](q,t) become increasingly pronounced on cooling. The corresponding dynamical correlation length ξ[sub 4](t) extracted from the small-q behavior of S[sub 4](q,t) provides an estimate of the range of correlated particle motion. We find that ξ[sub 4](t) has a maximum as a function of time t, and that the value of the maximum of ξ[sub 4](t) increases steadily from less than one particle diameter to a value exceeding nine particle diameters in the temperature range approaching the mode coupling temperature from above. At the maximum, ξ[sub 4](t) and the α relaxation time τ[sub α] are related by a power law. We also examine the individual contributions to g[sub 4](r,t), S[sub 4](q,t), and ξ[sub 4](t), as well as the corresponding order parameter Q(t) and generalized susceptibility χ[sub 4](t), arising from the self and distinct contributions to Q(t). These contributions elucidate key differences between domains of localized and delocalized particles.© 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structure and dynamics of aqueous solvation of ethanol and ethylene are studied by density functional theory based Car–Parrinello molecular dynamics. We did not find an enhancement of the structure of the hydrogen bonded network of hydrating water molecules. Both ethanol and ethylene can easily be accommodated in the hydrogen-bonded network of water molecules without altering its structure. This supports the conclusion from recent neutron diffraction experiments that there is no hydrophobic hydration around small hydrophobic groups. Analysis of the electronic charge distribution using Wannier functions shows that the dipole moment of ethanol increases from 1.8 D to 3.1 D upon solvation, while the apolar ethylene molecule attains an average dipole moment of 0.5 D. For ethylene, we identified configurations with π-H bonded water molecules, that have rare fourfold hydrogen-bonded water coordination, yielding instantaneous dipole moments of ethylene of up to 1 D. The results provide valuable information for the improvement of empirical force fields, and point out that for an accurate description of the aqueous solvation of ethanol, and even of the apoler ethylene, polarizable force fields are required. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The most common form of density functional calculations on molecular systems used generalized gradient approximation exchange-correlation functionals (such calculations can be applied to larger systems because no exact exchange is included). The most efficient and fastest such codes use an auxiliary basis set to fit the density so that only three-center integrals need to be evaluated. The codes DGAUSS and TURBOMOL use Gaussian basis sets, whereas the long-established ADF code uses Slater basis sets. We here examine the use of Slater basis sets. Our new code evaluates all required integrals numerically by quadrature. We report calculations on the G2 molecular set, contrasting them with similar calculations using Gaussian basis sets. Our conclusion, as far as energetics and structure are concerned, is that very similar predictions may be obtained from basis sets of the same size, and at approximately the same cost. [ABSTRACT FROM AUTHOR]

SiO[sub 2]-water clusters are studied using first-principles Born-Oppenheimer molecular dynamics based on density functional theory and generalized gradient approximations. Systematic investigations of structure and energetics as functions of cluster size demonstrate the roles of water molecules in chemical reactions. The water-assisted formation of a Si(OH)[sub 4] molecule from a single SiO[sub 2] molecule is revealed at the atomic level. The dynamics of dissociation of water molecules and formation of Si-OH bonds is investigated via simulations at finite temperature. A complex process that involves double and triple hydrogen atom transfer is discovered to be the reaction path. [ABSTRACT FROM AUTHOR]

Equilibrium self-assembly in fluids is studied in the framework of the lattice density-functional theory (DFT). In particular, DFT is used to model the phase behavior of anisotropic monomers. Though anisotropic monomers are a highly idealized model system, the analysis presented here demonstrates a formalism that can be used to describe a wide variety of phase transitions, including processes referred to as self-assembly. In DFT, the free energy is represented as a functional of order parameters. Minimization of this functional allows modeling spontaneous nano-scale phase transitions and self-assembly of supramolecular structures. In particular, this theory predicts micellization, lamellization, fluid–glass phase transitions, crystallization, and more. A classification of phase transitions based on general differences in self-assembled structures is proposed. The roles of dimensionality and intermolecular interactions in different types of phase transitions are analyzed. The concept of primordial codes is discussed in terms of the structural variety of self-assembled systems. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new formalism designed to treat long range interactions on surfaces, systems which are infinitely replicated in two spatial directions but have finite extent in the third, is developed. The new formalism is based in reciprocal space and, thus, permits the facile extension of standard plane-wave based density functional theory, Ewald summation, and smooth particle-mesh Ewald methods to handle surfaces efficiently. The method is tested on both model (body centered cubic lattices) and realistic problems (an ice surface with a defect and the 2 x 1 surface reconstruction of silicon) and found to be accurate, efficient, and a marked improvement on existing formulations in speed, accuracy, and utility. [ABSTRACT FROM AUTHOR]

Common density functional theories often use the so-called mean-field approximation to the attractive term of the free energy, which is not accurate when compared with molecular dynamics simulations. We applied a modification of this attractive free energy term of a local density functional approach where an analytical representation of the radial distribution function is taken into account. A cutoff Lennard-Jones-12-6 potential, divided according to the Weeks-Chandler-Andersen prescription, is used as the intermolecular interaction force model. This density functional perturbation approach gives the correct phase quilibrium and predicts a surface tension in excellent agreement with recent molecular dynamics simulations. The surface tension of some pure simple fluids is predicted from an intrinsic contribution, obtained by the modified density functional perturbation theory, plus contributions due to capillary wave roughening of the surface. The respective Lennard-Jones potential parameters were obtained from fitting to saturated liquid densities of the pure fluids. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Previous attempts to combine Hartree–Fock theory with local density-functional theory have been unsuccessful in applications to molecular bonding. We derive a new coupling of these two theories that maintains their simplicity and computational efficiency, and yet greatly improves their predictive power. Very encouraging results of tests on atomization energies, ionization potentials, and proton affinities are reported, and the potential for future development is discussed. [ABSTRACT FROM AUTHOR]

We derive expressions for molecular gradients and hessians for the case when the energy is evaluated using density functional theory. Although derivative expressions have been proposed previously, our derivation is based on the unitary exponential parameterization of the wavefunction, and our expressions are valid for local and non–local potentials. Density functional theory, although similar in implementation to standard SCF theory, differs in that it introduces an exchange–correlation term which is density dependent. The presence of such a quantity introduces additional derivative terms which are not present in standard approaches of electronic structure theory. Expressions are derived for both the exact Coulombic repulsion, as well as the case where the density is expressed as a fitted quantity. Given these choices our final equations offer a computationally tractable expression with particular emphasis on conditions which ensure that the computed quantities are numerically correct. We show that although the use of a fitted density allows significant computational savings in the energy and the first derivatives, it introduces additional computational complexity, beyond that normally encountered in traditional electronic structure methods, once second derivatives are evaluated. The evaluation of second derivatives also introduces derivatives of the exchange–correlation potential which have not been previously considered.The presence of such terms introduces the most serious computational complexity to the evaluation of any second derivative expression based on the density–functional formalism. Our derivation and analysis presents a computationally tractable procedure for the evaluation of all the terms required to compute the first and second derivatives with respect to perturbations such as nuclear coordinates, and external electric fields. Using a general set of response equations for the first order change in the wavefunction,... [ABSTRACT FROM AUTHOR]

A closed form for the chemical potentials of a fluid is presented that involves only integrals of the molecular distribution functions at the given state, (e.g., temperature and density). Thus no Kirkwood charging or thermodynamic integration is needed. An exact formula from a previous study is reanalyzed and a diagrammatical representation of the correlation functions involved is given. This representation involves, in addition to the usual total correlations, direct correlations, and the bridge function, B(r), a new star function, S(r). Analysis shows that the integral of the star function is the primitive of the bridge function, i.e., its functional derivative yields B(r). It is also related to the free-energy functional F[ρ] in density-functional theories for nonuniform systems. Methods for estimating the star function are given. Tests on uniform hard-sphere fluid are carried out to demonstrate the new formulas. We have examined several current closures: the Percus–Yevick, Martynov–Sarkisov, Ballone–Pastore–Galli–Gazzillo, and a Verlet-modified (VM) closure. The VM approach gives the best reproduction of the bridge function. Much improved results are obtained for the chemical potentials of hard spheres at densities ρd3 ranging from 0.3 to 0.85. [ABSTRACT FROM AUTHOR]

Examines a one-dimensional model with molecular interactions favoring the formation of clusters with a defined optimal size. Effects of increasing the density at low temperature on the system; Similarity with amphiphilic systems; Comparison between exact results and the mean field density functional approximation.

Discusses the use of density functional theory (DFT) methods to determine the molecular structures and total energies of perflurocarbons. Adiabatic electron affinity; Vertical electron affinity; Vertical detachment energy of the neutral-anion.

Reports on density functional calculations of molecular g-tensors in the zero-order regular approximation for relativistic effects. Consideration of spin-orbit coupling using the zero-order regular approximation to the Dirac equation; Solution for the problem of gauge dependence.

The dynamics of clusters (H2O)nH+ (n=1,2,3,4) interacting with an NH3 molecule has been studied by first-principles Born–Oppenheimer molecular dynamics (BOMD) simulations. These small clusters are chosen as prototype systems for studying the mechanisms of proton transfer at atomistic level. We focus on the fundamental steps of proton motion in molecular clusters, the dynamical consequences of proton affinities, and the interplay between proton motion and proton affinity in these systems. A characteristic feature of the motion, the forming and breaking of O–H bonds in H3O+ is analyzed in detail. The transfer process is found to be consecutive along a quasi-one-dimensional channel. The umbrella mode in NH3 can easily be excited to direct the lone pair of the ammonia molecule to the water clusters. The hydronium ion, however, reorients mainly via rotation. When NH3 reaches one terminal water molecule of a protonated water cluster, the system undergoes a series of intermediate states in which the mobile protons travel within the water clusters, H3O+ transients are formed as protons approach individual water molecules. The lifetime of the H3O+ transient is 8–20 fs, or 1–3 vibrational periods of the O–H stretch mode. Proton transfer is observed for n=1, 2, 3, although for n=3 NH+4(H2O)3 is in existence with NH3(H2O)2H+. For n=4, NH3(H2O)4H+ is the dominant statistical configuration. Vibrational spectrum of NH3(H2O)4H+ is analyzed in detail. The features of the spectrum can be used, in principle, to probe the proton motion in the transition state region reactions. In these calculations, the electronic charge distribution is calculated concurrently with the nuclear dynamics. An analysis of isocharge density surfaces gives qualitative and quantitative descriptions of the dynamics of electronic redistribution. The BOMD is performed in the framework of density functional theory with local spin density and generalized gradient... [ABSTRACT FROM AUTHOR]

Three frequently used gradient-corrected density functionals (B, BP, and BLYP) are applied in an ab initio molecular dynamics simulation of liquid water in order to evaluate their performance for the description of condensed aqueous systems. A comparison of structural characteristics (radial distribution functions) and dynamical properties (vibrational spectra, orientational relaxation, and self-diffusion) leads to the conclusion that hydrogen bonding is too weak in the usual local density approximation corrected for exchange only according to Becke (B), whereas adding the gradient correction for correlation according to Perdew (BP) yields effective hydrogen bonds in the liquid that are too strong. The combination of B with the semilocal correlation functional according to Lee, Yang, and Parr (BLYP) yields the best agreement with experiment. The computational method, which is the basis for the determination of (adiabatic) electronic structure in the ab initio molecular dynamics simulation, has been validated by an extensive series of test calculations for the water dimer, which will also be presented here. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Results of molecular dynamics simulations (MDS) of o-terphenyl, a glass-forming liquid, are analyzed in terms of the coupling model of relaxation. At low temperatures thermally activated relaxation processes are suppressed, whereby the density–density correlation function, C(t), obtained by MDS is determined entirely by vibrational modes. This enables the low temperature data to be used to deduce the vibrational density of states, g(ω). With g(ω) determined, the vibrational contribution, Cpho(t), is calculated at higher temperatures assuming that g(ω) is independent of temperature. At higher temperatures, relaxation makes its appearance and is modeled here by the fast dynamics of the coupling model. Assuming that vibration and relaxation contribute independently, the density–density self-correlation function is given by the product Cpho(t)Crel(t), with the relaxation part obtained from the coupling model. There is good overall agreement between the calculated C(t) and the MDS data. Microscopic parameters, including the energy barrier for reorientation of the o-terphenyl molecule, are extracted from the MDS results. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new formalism for quantum functional sensitivity analysis (QFSA) of atom–diatom reactions in the gas phase is developed within a version of Manolopoulos et al.’s [J. Chem. Phys. 93, 403 (1990)] log-derivative Kohn variational method containing contracted translational basis functions. A reference energy, Emid, is introduced to define boundary translational functions which completely remove all scattering energy dependence from the basis functions. This greatly facilitates scattering calculations for a range of energies about Emid without having to recalculate any of the so-called ‘‘stiffness’’ matrix elements. Our new approach to QFSA is applied to study the sensitivity of the H+H2, D+H2, and H+D2 reaction probabilities to the Boothroyd–Keogh–Martin–Peterson (BKMP) potential energy surface. The transition probability sensitivities of both D+H2 and H+D2 are very similar to those of H+H2 at low energies, but at higher energies, the sensitivities of the H+D2 reaction differ from those of the other two isotopomers. Isotopomers that have very similar reaction probability profiles also have very similar sensitivities to the potential. All three isotopomers exhibit a large region of positive sensitivity at the top of the barrier for an approximately 0.1 to 0.2 eV energy range above threshold. For these energies (∼0.3 to 0.5 eV above the barrier), it is possible to increase the reaction probability with slight increases in barrier height. Sensitivity results from our new code have also provided a wealth of information about (i) how small, localized changes in the potential affect product state distributions, resonance features, and reactivity; and (ii) where the dynamics is most sensitive to inaccuracies in the potential. Comparisons are also made of transition probabilities and sensitivities on the BKMP surface to those of other potential energy surfaces. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Instantaneous normal mode calculations of the low frequency solvent modes of carbon tetrachloride (CCl4) and chloroform (CHCl3), and experiments on the vibrational population dynamics of the T1u CO stretching mode (∼1980 cm-1) of tungsten hexacarbonyl in CCl4 and CHCl3 are used to understand factors affecting the temperature dependence of the vibrational lifetime. Picosecond infrared pump–probe experiments measuring the vibrational lifetime of the T1u mode from the melting points to the boiling points of the two solvents show a dramatic solvent dependence. In CCl4, the vibrational lifetime decreases as the temperature is increased; however, in CHCl3, the vibrational lifetime actually becomes longer as the temperature is increased. The change in thermal occupation numbers of the modes in the solute/solvent systems cannot account for this difference. Changes in the density of states of the instantaneous normal modes and changes in the magnitude of the anharmonic coupling matrix elements are considered. The calculated differences in the temperature dependences of the densities of states appear too small to account for the observed difference in trends of the temperature dependent lifetimes. This suggests that the temperature dependence of the liquid density causes significant changes in the magnitude of the anharmonic coupling matrix elements responsible for vibrational relaxation. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]