Ab Initio Potential Research Articles

Compound-model morphed potential for the hydrogen bond HCN⋯HF

ABSTRACTA 5-D compound-model morphed potential has been generated for the prototype hydrogen-bonded dimer HCN⋯HF. The potential includes the intermolecular degree of freedom and the HF stretching vibration. Only five morphing parameters are optimised, correcting for inadequacies in the underlying ab initio potentials. Band origin fundamental vibrational frequencies are fitted to an average absolute error of 0.01 cm−1. The calculated value of the ground state dissociation energy, D0 = 1969 cm−1, is in excellent agreement with the experimental value of 1970(10) cm−1 [Oudejans and Miller, Chem. Phys. 239 (1998) 345]. Additional vibrational frequencies are predicted with an average absolute error of 1.61 cm−1. Application of the morphing methodology to characterise hydrogen-bonded complexes is discussed.

Influence of quantum intermolecular interaction on internal flows of rarefied gases

In order to model gaseous flows over the whole temperature range beginning from 1 K, the intermolecular interaction should be considered on the basis of quantum approach. Such a consideration becomes important in case of light gases like helium and hydrogen. Recently, the direct simulation Monte Carlo (DSMC) method widely used to calculate flows of gases has been generalized to implement the quantum approach to intermolecular collisions. To evaluate the influence of the quantum scattering on typical flows of light gases, a benchmark problem has been solved for two helium isotopes 3He and 4He using an ab initio potential. More specifically, the flow-rate and flow-field of helium flowing through an orifice have been calculated over the temperature range from 1 K to 300 K for various values of the pressure ratio with the numerical error of 0.5%. As expected, no influence of the quantum effects on the flow-rate has been detected for the temperature 300 K. Though, the quantum approach requires less computational effort than the classical one at this temperature. For temperatures lower than 300 K, the influence of the quantum effects exceed the numerical error and reaches 41% at the temperature of 3 K. In this case, the quantum interaction is the only approach to model gas flows.

Structure of planar shock waves in gaseous mixtures based on ab initio direct simulation

The structure of planar shock wave propagating through a helium–argon mixture is modeledby the direct simulation Monte Carlo (DSMC) method based on ab initio potentials for a wide range of the Mach number and for various molar fractions. The use of ab initio potentials allows us to carry out the simulation without any adjustable parameter usually extracted from experimental data. As a result, the density, temperature, molar fraction, diffusion velocity, pressure tensor and heat flow profiles inside of the shock wave are calculated. The temperature overshoot phenomenon is discussed in details. The slopes of density are calculated with the numerical error less than 0.5 %. It is pointed out that the slopes for mixture are always smaller than those for a single gas.

Modeling of transport phenomena in gases based on quantum scattering

A quantum interatomic scattering is implemented in the direct simulation Monte Carlo (DSMC) method applied to transport phenomena in rarefied gases. In contrast to the traditional DSMC method based on the classical scattering, the proposed implementation allows us to model flows of gases over the whole temperature range beginning from 1 K up any high temperature when no ionization happens. To illustrate the new numerical approach, two helium isotopes 3He and 4He were considered in two canonical problems, namely, heat transfer between two planar surfaces and planar Couette flow. To solve these problems, the ab initio potential for helium is used, but the proposed technique can be used with any intermolecular potential. The problems were solved over the temperature range from 1 K to 3000 K and for two values of the rarefaction parameter δ=1 and 10. The former corresponds to the transitional regime and the latter describes the temperature jump and velocity slip regime. No influence of the quantum effects was detected within the numerical error of 0.1% for the temperature 300 K and higher. However, the quantum approach requires less computational effort than the classical one in this temperature range. For temperatures lower than 300 K, the influence of the quantum effects exceed the numerical error and reaches 67% at the temperature of 1 K. Numerical data provided in Supplemental Material can be used to model any kind of helium flow at any temperature.

Cost-Effective Method for Free-Energy Minimization in Complex Systems with Elaborated Ab Initio Potentials.

We describe a method to locate stationary points in the free-energy hypersurface of complex molecular systems using high-level correlated ab initio potentials. In this work, we assume a combined QM/MM description of the system although generalization to full ab initio potentials or other theoretical schemes is straightforward. The free-energy gradient (FEG) is obtained as the mean force acting on relevant nuclei using a dual level strategy. First, a statistical simulation is carried out using an appropriate, low-level quantum mechanical force-field. Free-energy perturbation (FEP) theory is then used to obtain the free-energy derivatives for the target, high-level quantum mechanical force-field. We show that this composite FEG-FEP approach is able to reproduce the results of a standard free-energy minimization procedure with high accuracy, while simultaneously allowing for a drastic reduction of both computational and wall-clock time. The method has been applied to study the structure of the water molecule in liquid water at the QCISD/aug-cc-pVTZ level of theory, using the sampling from QM/MM molecular dynamics simulations at the B3LYP/6-311+G(d,p) level. The obtained values for the geometrical parameters and for the dipole moment of the water molecule are within the experimental error, and they also display an excellent agreement when compared to other theoretical estimations. The developed methodology represents therefore an important step toward the accurate determination of the mechanism, kinetics, and thermodynamic properties of processes in solution, in enzymes, and in other disordered chemical systems using state-of-the-art ab initio potentials.

IR Spectra of (HCOOH)2 and (DCOOH)2: Experiment, VSCF/VCI, and Ab Initio Molecular Dynamics Calculations Using Full-Dimensional Potential and Dipole Moment Surfaces.

We report quantum VSCF/VCI and ab initio molecular dynamics (AIMD) calculations of the IR spectra of (HCOOH)2 and (DCOOH)2, using full-dimensional, ab initio potential energy and dipole moment surfaces (PES and DMS). These surfaces are fits, using permutationally invariant polynomials, to 13 475 ab initio CCSD(T)-F12a electronic energies and MP2 dipole moments. Here "AIMD" means using these ab initio potential and dipole moment surfaces in the MD calculations. The VSCF/VCI calculations use all (24) normal modes for coupling, with a four-mode representation of the potential. The quantum spectra align well with jet-cooled and room-temperature experimental spectra over the spectral range 600-3600 cm-1. Analyses of the complex O-H and C-H stretch bands are made based on the mixing of the VSCF/VCI basis functions. The comparisons of the AIMD IR spectra with both experimental and VSCF/VCI ones provide tests of the accuracy of the AIMD approach. These indicate good accuracy for simple bands but not for the complex O-H stretch band, which is upshifted from experimental and VSCF/VCI bands by roughly 300 cm-1. In addition to testing the AIMD approach, the PES, DMS, and VSCF/VCI calculations for formic acid dimer provide opportunities for testing other methods to represent high-dimensional data and other methods that perform postharmonic vibrational calculations.

Photodissociation of CS from Excited Rovibrational Levels

Abstract Accurate photodissociation cross sections have been computed for transitions from the X 1Σ+ ground electronic state of CS to six low-lying excited electronic states. New ab initio potential curves and transition dipole moment functions have been obtained for these computations using the multi-reference configuration interaction approach with the Davidson correction (MRCI+Q) and aug-cc-pV6Z basis sets. State-resolved cross sections have been computed for transitions from nearly the full range of rovibrational levels of the X 1Σ+ state and for photon wavelengths ranging from 500 Å to threshold. Destruction of CS via predissociation in highly excited electronic states originating from the rovibrational ground state is found to be unimportant. Photodissociation cross sections are presented for temperatures in the range between 1000 and 10,000 K, where a Boltzmann distribution of initial rovibrational levels is assumed. Applications of the current computations to various astrophysical environments are briefly discussed focusing on photodissociation rates due to the standard interstellar and blackbody radiation fields.

Interaction potentials and transport properties of Ba, Ba+, and Ba2+ in rare gases from He to Xe.

A highly accurate, consistent set of ab initio interaction potentials is obtained for the title systems at the coupled cluster with singles, doubles, and non-iterative triples level of theory with extrapolation to the complete basis set limit. These potentials are shown to be more reliable than the previous potentials based on their long-range behavior, equilibrium properties, collision cross sections, and transport properties.

Quantum Second Virial Coefficients for Krypton -86 Gas

A quantum second virid coefficient for krypton gas is calculated using ab initio potential, Barker et al potential and Morse potential. Calculations have been made using Galitskii-Migdal-Feynman (GMF) formalism. Agreements with experiments is good though not perfect.

Ab Initio Simulation of Transport Properties in Rb–CH4 and Cs–CH4 Laser Media

The pair interaction potentials of the weakly bound Rb–CH4 and Cs–CH4 systems, which are active media of alkali metal vapor lasers with broadband diode or excimer laser pumping, were calculated by the ab initio method. The electronic problem was solved by the coupled-cluster method in the CCSD(T) version including the basis set superposition error and extrapolation to an infinite basis set. The obtained pointwise ab initio potentials were approximated by the analytical functions based on the orthogonal Chebyshev polynomial expansion with correct asymptotic behavior at the dissociation limit and then used within the framework of the molecular kinetic theory of rarefied gases to evaluate the reduced collision integrals and mutual diffusion coefficients.

Ab initio optical potentials and nucleon scattering on medium mass nuclei

We show first results for the elastic scattering of neutrons off oxygen and calcium isotopes obtained from ab initio optical potentials. The potential is derived using self–consistent Green’s function theory (SCGF) with the saturating chiral interaction NNLOsat. Calculations are compared to available scattering data and show that it is possible to reproduce low energy scattering observables in medium mass nuclei from first principles.

Coupled-channel study of the Rydberg-valence interaction in HBr.

We report an ab initio study of the low-lying valence and Rydberg states of HBr. The calculations are carried out employing the multireference single- and double-excitation configuration interaction method including the spin-orbit interaction. The first excited adiabatic potential of 1Σ+ symmetry presents two minima which correspond to the Rydberg E1Σ+ and valence V1Σ+ observed states. We calculate the vibrational levels of these two states using a coupled-channel treatment based on the two diabatic potentials deduced from the ab initio adiabatic potentials and the Rydberg-valence interaction. The chaotic energy separations between the observed levels are well reproduced in the calculations. We have also obtained for the first time theoretical data for numerous Rydberg states of HBr lying in the (66-79) × 103 cm-1 excitation energy interval. The calculated spectroscopic parameters are found to be in good agreement with experiment and provide a basis for future studies of radiative and non-radiative processes in the HBr molecule.

Zeolite Adsorption Free Energies from ab Initio Potentials of Mean Force.

The ability of metal-exchanged zeolites to chemisorb small gas molecules is key to their performance as heterogeneous catalysts and gas-separating agents. Here, we propose and evaluate an ab initio potential of mean force (PMF) method for computing adsorption free energies of representative small molecules to Cu-exchanged sites in SSZ-13 zeolite. We show that Cu ions are mobilized by adsorbates and, as a result, computed free energies are significantly more negative than those obtained from a conventional harmonic oscillator model. PMF-derived free energies are consistent with available experiment and, in many cases, with a dynamics-based quasi-harmonic analysis (QHA). The PMF approach avoids the artificial partitioning of degrees of freedom intrinsic to the QHA. On the basis of the PMF results, we propose a simple correlation to estimate free energies from computed adsorption energies and gas-phase entropies.

Electron affinity of liquid water

Understanding redox and photochemical reactions in aqueous environments requires a precise knowledge of the ionization potential and electron affinity of liquid water. The former has been measured, but not the latter. We predict the electron affinity of liquid water and of its surface from first principles, coupling path-integral molecular dynamics with ab initio potentials, and many-body perturbation theory. Our results for the surface (0.8 eV) agree well with recent pump-probe spectroscopy measurements on amorphous ice. Those for the bulk (0.1–0.3 eV) differ from several estimates adopted in the literature, which we critically revisit. We show that the ionization potential of the bulk and surface are almost identical; instead their electron affinities differ substantially, with the conduction band edge of the surface much deeper in energy than that of the bulk. We also discuss the significant impact of nuclear quantum effects on the fundamental gap and band edges of the liquid.

Accurate virial coefficients of gaseous krypton from state-of-the-art ab initio potential and polarizability of the krypton dimer.

We have developed a new krypton-krypton interaction-induced isotropic dipole polarizability curve based on high-level ab initio methods. The determination was carried out using the coupled-cluster singles and doubles plus perturbative triples method with very large basis sets up to augmented correlation-consistent sextuple zeta as well as the corrections for core-core and core-valence correlation and relativistic effects. The analytical function of polarizability and our recently constructed reference interatomic potential [J. M. Waldrop et al., J. Chem. Phys. 142, 204307 (2015)] were used to predict the thermophysical and electromagnetic properties of krypton gas. The second pressure, acoustic, and dielectric virial coefficients were computed for the temperature range of 116 K-5000 K using classical statistical mechanics supplemented with high-order quantum corrections. The virial coefficients calculated were compared with the generally less precise available experimental data as well as with values computed from other potentials in the literature {in particular, the recent highly accurate potential of Jäger et al. [J. Chem. Phys. 144, 114304 (2016)]}. The detailed examination in this work suggests that the present theoretical prediction can be applied as reference values in disciplines involving thermophysical and electromagnetic properties of krypton gas.

Symmetry-adapted perturbation theory potential for the adenine dimer.

A new intermolecular interaction potential for the adenine dimer has been developed with the help of a combination of symmetry-adapted perturbation theory and density functional theory (DFT-SAPT). Supermolecular intermolecular interaction energy computations on hydrogen-bonded and stacked adenine dimers at B3LYP-D, MP2, SCS-MP2, SCS-MI-MP2 and CCSD(T) levels showed that DFT-SAPT is in a very good agreement with CCSD(T). The developed ab initio intermolecular potential has been used to predict the cluster structure of adenine dimers, trimers and tetramers. These global cluster optimizations reproduced adenine dimers reported in the literature and moreover new low-energy structures were also located. For trimers and tetramers, new hydrogen-bonded and stacked low-energy structures have also been found. The current findings suggest that the new ab initio potential can further be exploited to reveal the structure and energy of much larger supramolecular adenine clusters.

Fully quantum calculation of the second and third virial coefficients of water and its isotopologues from ab initio potentials.

Path-Integral Monte Carlo methods were applied to calculate the second, B(T), and the third, C(T), virial coefficients for water. A fully quantum approach and state-of-the-art flexible-monomer pair and three-body potentials were used. Flexible-monomer potentials allow calculations for any isotopologue; we performed calculations for both H2O and D2O. For B(T) of H2O, the quantum effect contributes 25% of the value at 300 K and is not entirely negligible even at 1000 K, in accordance with recent literature findings. The effect of monomer flexibility, while not as large as some claims in the literature, is significant compared to the experimental uncertainty. It is of opposite sign to the quantum effect, smaller in magnitude than the latter below 500 K, and varies from 2% at 300 K to 10% at 700 K. When monomer flexibility is accounted for, results from the CCpol-8sf pair potential are in excellent agreement with the available experimental data and provide reliable B(T) values at temperatures outside the range of experimental data. The flexible-monomer MB-pol pair potential yields B(T) values that are slightly too high compared to experiment. For C(T), our calculations confirm earlier findings that the use of three-body potential is necessary for meaningful predictions. However, due to various uncertainties of the potentials used, especially the three-body ones, we were not able to establish benchmark values of C(T), although our results are in qualitative agreement with available experimental data. The quantum effect, never before included for water, reduces the magnitude of the classical value for H2O by a factor of 2.5 at 300 K and is not entirely negligible even at 1000 K.

Ab initio теория уравнения состояния сжатых кристаллов инертных газов

AbstractNonempirical equations of state of compressed rare gas crystals Ne, Ar, Kr, and Xe are studied on the basis of the earlier-obtained ab initio adiabatic potential. The paired and three-body short-range repulsive potentials are calculated by the Hartree–Fock method in the basis of localized functions with their exact mutual orthogonalization and do not contain experimentally determined parameters. The theory is compared with the experiment and results of calculations by other authors. Analysis of the proposed equations of state for large compressions has shown the importance of taking into account the three-body interaction and the terms of the higher order in the overlap integral in compressed neon and the sufficiency of the quadratic approximation in the orthogonalization of functions in heavy rare gas crystals.

Ab initio theory of the equation of state for compressed rare gas crystals

Nonempirical equations of state of compressed rare gas crystals Ne, Ar, Kr, and Xe are studied on the basis of the earlier-obtained ab initio adiabatic potential. The paired and three-body short-range repulsive potentials are calculated by the Hartree–Fock method in the basis of localized functions with their exact mutual orthogonalization and do not contain experimentally determined parameters. The theory is compared with the experiment and results of calculations by other authors. Analysis of the proposed equations of state for large compressions has shown the importance of taking into account the three-body interaction and the terms of the higher order in the overlap integral in compressed neon and the sufficiency of the quadratic approximation in the orthogonalization of functions in heavy rare gas crystals.

Thermal transport and microscopic dynamics in filled skutterudite YbFe4Sb12 studied by ab initio molecular dynamics simulation

Filled skutterudite is a typical thermoelectric material with high thermoelectric figure of merit at intermediate temperatures. One of the important features is the low lattice thermal conductivity (L) caused by the low frequency vibrations of filler atoms in the oversized void cages. In the past decades, it has been still under debate whether the underlying phonon scattering mechanism should be considered to be resonant scattering or enhanced three-phonon process. To clarify the role played by the filler atoms in reducing the lattice thermal conductivity, we study the microscopic dynamical process of filler and related interactions by means of ab initio molecular dynamics (AIMD) and temperature dependent effective potential (TDEP) technique. Firstly, we simulate the dynamical trajectories of fully filled skutterudite YbFe4Sb12 at different temperatures through AIMD. In this approach, the nonlinear guest-host interactions at finite temperatures are taken into consideration naturally from dynamical trajectories. Then, we extract the effective temperature-dependent harmonic and anharmonic interatomic force constants (IFCs) by TDEP method through the statistical analyses of both trajectories and forces. The atomic participation ratios and lifetimes of phonon modes are calculated based on the effective IFCs. The results demonstrate that the local vibration modes of Yb couple with acoustic branches and reduce the lifetimes of the lattice phonons significantly. However, the calculated L, which is on the assumption that the filler interacts with lattice phonons through three-phonon collision, still deviates from the experimental result. In order to rationalize the discrepancy, we analyze the correlation properties between different Yb atoms by velocity coherence in atomic dynamical motions. The localized and independent vibration characteristic of Yb is found in this analysis. This implies that the motions of Yb atoms deviate from the periodic and collective vibration excitation paradigm of phonon. Therefore, the mechanism for how filler atoms scatter lattice phonon and enhance thermal resistance is beyond three-phonon scattering process. We thus introduce resonant scattering into the lifetimes of Yb-dominant localized vibration modes, and so-calculated L is in a good agreement with the experimental data. Overall, we come to a conclusion that both the phonon-phonon interaction and the resonant scattering due to the localized oscillators cause the low lattice thermal conductivity of YbFe4Sb12.