Accurate Ab Initio Potential Research Articles

Accurate ab initio potential for argon dimer including highly repulsive region

An ab initio potential has been developed for the argon dimer. This potential is based on coupled-cluster calculations with single, double, and non-iterated triple excitations in a sequence of very large basis sets, up to augmented sextuple-zeta quality and containing bond functions, followed by extrapolations to the complete basis set limit. The calculations included intermolecular distances as small as 0.25 Å, where the interaction potential is of the order of 4 keV. The computed points were fitted by an analytic expression. The new potential has the minimum at 3.767 Å with a depth of 99.27 cm−1 respectively, very close to experimental values of 3.761 ± 0.003 Å and 99.2 ± 1.0 cm−1 respectively. The potential was used to compute the spectra of the argon dimer and the virial coefficients. The latter calculations suggest a possible revision of the established experimental reference results. From the agreement achieved with experimental values and from comparisons of the fit with available piecewise information on specific regions of the argon–argon interaction, one can assume that the present work provides the best overall representation of the true argon–argon potential to date.

Molecular simulation of the shear viscosity and the self-diffusion coefficient of mercury along the vapor-liquid coexistence curve

In earlier work [G. Raabe and R. J. Sadus, J. Chem. Phys. 119, 6691 (2003)] we reported that the combination of an accurate two-body ab initio potential with an empirically determined multibody contribution enables the prediction of the phase coexistence properties, the heats of vaporization, and the pair distribution functions of mercury with reasonable accuracy. In this work we present molecular dynamics simulation results for the shear viscosity and self-diffusion coefficient of mercury along the vapor-liquid coexistence curve using our empirical effective potential. The comparison with experiment and calculations based on a modified Enskog theory shows that our multibody contribution yields reliable predictions of the self-diffusion coefficient at all densities. Good results are also obtained for the shear viscosity of mercury at low to moderate densities. Increasing deviations between the simulation and experimental viscosity data at high densities suggest that not only a temperature-dependent but also a density-dependent multibody contribution is necessary to account for the effect of intermolecular interactions in liquid metals. An analysis of our simulation data near the critical point yields a critical exponent of beta = 0.39, which is identical to the value obtained from the analysis of the experimental saturation densities.

Monte Carlo simulations of vapor–liquid equilibria of neon using an accurate ab initio pair potential

Gibbs ensemble Monte Carlo (GEMC) simulations are reported for the vapor–liquid phase coexistence of neon using an accurate ab initio pair potential (NE3) and also a quantum effective Wigner–Kirkwood potential (NE3-WK) based on the NE3 potential. It is noticed that the results from simulations for the thermodynamic properties of phase equilibria deviate from the experimental values (the simulated curve is outside the experimental temperature–density phase envelope). With an ab initio three-body potential of limited quality, the thermodynamic properties of phase equilibria are estimated in good agreement with experiment. It should be stressed that in contrast to most other work no empirical data are used in the present work.

Quantitative spectroscopic and theoretical study of the optical absorption spectra of H2O, HOD, and D2O in the 125-145 nm region.

The room temperature absorption spectra of water and its isotopomers D2O and HOD have been determined in absolute cross section units in the 125 to 145 nm wavelength region using synchrotron radiation. The experimental results for these B band spectra are compared with results from quantum mechanical calculations using accurate diabatic ab initio potentials. A Monte Carlo sampling over the initial rotational states of the molecules is applied in order to calculate the cross sections at a temperature of 300 K. The overall rotation of the water molecule is treated exactly. Both for the experimental and for the theoretical spectrum an analysis is made in terms of a component attributed to rapid direct dissociation processes and a component attributed to longer-lived resonances. The agreement between the results from experiment and theory is excellent for H2O and D2O. In the case of HOD in the results of theory two more resonances are found at low energy. It is demonstrated that the width of the resonances of 0.04 eV is the result of overlapping and somewhat narrower resonances in the spectra of molecules differing in rotational ground state.

Molecular simulation of the vapor–liquid coexistence of mercury

The vapor–liquid coexistence properties of mercury are determined from molecular simulation using empirical intermolecular potentials, ab initio two-body potentials, and an effective multibody intermolecular potential. Comparison with experiment shows that pair-interactions alone are inadequate to account for the vapor–liquid coexistence properties of mercury. It is shown that very good agreement between theory and experiment can be obtained by combining an accurate two-body ab initio potential with the addition of an empirically determined multibody contribution. As a consequence of this multibody contribution, we can reliably predict mercury’s phase coexistence properties and the heats of vaporization. The pair distribution function of mercury can also be predicted with reasonable accuracy.

Ab initio three-body interactions for water. I. Potential and structure of water trimer

A new ab initio three-body potential for water has been generated from the Hartree–Fock method and symmetry-adapted perturbation theory calculations performed at 7533 trimer geometries. The calculated nonadditive energies were then fitted to a physically motivated analytic formula containing representations of short-range exchange contributions and damped induction terms. To our knowledge, this is the first time the short-range nonadditive interactions have been explicitly included in a potential for water. The fitted nonadditive potential was then applied, together with an accurate ab initio pair potential, SAPT-5s, to evaluate the effects of nonadditivity on the structure and energetics of water trimer.

Calculation of Thermodynamical, Transport and Structural Properties of Neon in Liquid and Supercritical Phases by Molecular Dynamics Simulations Using an Accurate ab initio Pair Potential

An analytical potential energy curve (NE3) is constructed from points obtained by accurate ab initio calculations. The quality of the pair potential is established by a comparison of calculated and experimental second virial coefficients as a function of temperature. Molecular dynamics equilibrium simulations are performed with the NE3 potential for pressures between 20 and 1000 MPa and temperatures between 100 and 600 K in the supercritical phase and for one point in the liquid phase of neon. The properties are compared with those obtained from experiment. It is found that the accurate pair potential has a large effect on pressure, energies and enthalpies and a significant influence on other thermodynamic properties but little influence on transport and structural properties. In the supercritical phase the deviation between the calculated quantum-corrected and experimental pressure values is always less than 2%.

Host–guest charge transfer states: CN doped Kr and Xe

The host–guest charge transfer absorption of CN doped krypton and xenon matrices are identified through direct analogy with the previously assigned transitions of Cl/Kr and Cl/Xe. These intense, structured absorption bands appear with the onset at 245 nm in Kr and 360 nm in Xe. Excitation of the CN/Kr charge transfer band at 193 nm leads to emission over CN(A(2Π)→X(2Σ)) transition, indicating that an efficient curve crossing precludes the ionic state from radiating. No emissions were seen in CN/Xe when excited at 193 nm. The charge transfer absorption spectrum of CN/Kr is reproduced through an extended diatomics-in-ionic-systems treatment, using accurate ab initio pair potentials and transition dipoles as input, without further adjustment. The delocalized hole states are then analyzed in real-space, using atomic bases distributed over as many as eleven shells surrounding the CN− center. The ionic states are well described as J=1/2, 3/2 valence bands bound to CN−, with a substructure that cannot be exclusively assigned to a single quantum number. The strong absorptions terminate on states in which 70%–95% of the hole density remains on the first nearest neighbor shell, with hole densities of 1%–5% extended out to R=8 Å. In higher ionic states, with weaker transition dipoles, the hole density maximizes on shells removed by 10 Å from the ionic center. Although these delocalized states provide channels for charge separation via self-trapping of holes, save for a weak signal from the impurity trapped hole at H+ centers, the experiments do not provide evidence for significant charge separation.

One-electron pseudopotential calculations of excited states of LiAr, NaAr, and KAr

The potential curves and spectroscopic constants of the excited states of alkali–argon diatomics MRg (M=Li, Na and K, Rg=Ar) are calculated using usual semilocal single valence electron pseudopotentials on alkali atoms [M+]-core pseudopotentials), semilocal pseudopotentials replac(ing all the electrons of argon ([Ar]-core pseudopotentials), and core polarization pseudopotentials on both centers. All states dissociating into Li(2s, 2p, 3s, 3p, 3d, 4s, and 4p), Na(3s, 3p, 3d, 4s, 4p, 4d, 5p) and K(4s, 4p, 5s, 3d, 5p, 4d, 6s, 4f, 6p, 5d, 7s, 5f) are considered. The core–core interactions for Li+Ar and Na+Ar are included using the accurate ab initio potentials of Ahmadi et al. [G. R. Ahmadi, J. Almlöf, and I. Roeggen, Chem. Phys. 199, 33 (1995); G. R. Ahmadi and I. Roeggen, J. Phys. B 27, 5603 (1994)] while the K+Ar ion data are determined by MP2 all-electron calculations.

Prediction of the Gibbs energies and an improved equation of state for water at extreme conditions from ab initio energies with classical simulations

The hydration free energies of water at two state points (2344 K and 1.0 g/cm3, and 2000 K and 1.8 g/cm3) were calculated using the ABC-FEP method (Wood et al., 1999) that combines interaction energies calculated from ab initio quantum mechanics with configurations sampled from classical simulations. We start with a calculation of the Gibbs energy of an approximate classical water model by a standard simulation method. Next, the difference in Gibbs energy of the approximate model potential and more accurate ab initio interaction potential was obtained using the free energy perturbation method. At 2000 K and 1.8 g/cm3, it was necessary to derive a better classical model from the ab initio results due to the inaccuracy of the classical model at short distances. The estimated accuracy of the predicted free energies is ±15 kJ/mol. The calculated results are used to examine the predictive capability of previously proposed classical simulations and equations of state. The equation of Pitzer and Sterner (1994) was reparameterized using the available experimental data and the newly calculated data points.

Theoretical prediction of the ArO − anion ZEKE photoelectron spectrum

High-resolution zero electron kinetic energy spectra of the weakly bound ArO − anion are predicted using the method which combines atoms-in-molecule and Rau–Fano models for calculating relative intensities and accurate ab initio interaction potentials. ZEKE spectrum envelope and its temperature variation are shown to be essentially determined by the electronic photodetachment probabilities which are highly selective with respect to the symmetry of the spin–orbit components of anion and neutral electronic states.

Monte Carlo simulations of neon and argon using ab initio potentials

Gibbs ensemble Monte Carlo simulations of neon and argon have been performed with pair potentials taken from literature as well as with new ab initio potentials from just above the triple point to close to the critical point. The densities of the coexisting phases, their pair correlation functions, the vapour pressure and the enthalpy and entropy of vaporization have been calculated. The influence of the potential choice and of the addition of the Axilrod-Teller (AT) three-body potential on the above mentioned properties have been investigated. It turns out that an accurate ab initio two-body potential in connection with the AT potential yields very good results for thermodynamic properties of phase equilibria.

Tunnelling lifetimes of the rovibronic levels in the B electronic state of the CH radical obtained from ab initio data

Rovibronic spectroscopic parameters have been obtained for the X and B states of the CH radical using accurate ab initio potentials. The potentials have been calculated with the multi-reference averaged quadratic coupled cluster (MR-AQCC) method using a large reference space and the cc-pVQZ basis. The one-dimensional Schrödinger equation of the nuclei has been solved exactly to get the vibrational—rotational energy levels. The energies and tunnelling predissociation widths of quasibound levels on these potentials have been calculated, plus the Franck—Condon factors for many of the rovibronic transitions.

Ab initio calculation of the frequency-dependent interaction induced hyperpolarizability of Ar2

The frequency-dependent interaction induced polarizability and second hyperpolarizability of the argon dimer are computed for a range of internuclear distances employing the coupled cluster singles and doubles response approach. The frequency dependence of the interaction-induced properties is treated through second order in the frequency arguments using expansions in Cauchy moments and hyperpolarizability dispersion coefficients. The dielectric, the refractivity, the Kerr and the hyperpolarizability second virial coefficients are computed for a range of temperatures employing a recent accurate ab initio potential for the ground state of the argon dimer. For most of the computed virial coefficients good agreement is obtained between the present ab initio results and the available experimental data.

Calculation of resonance states of non-rotating HOCl using an accurate ab initio potential

Complex L2 calculations for selected resonance states of non-rotating HOCl are reported using a recently developed, accurate abinitio potential energy surface [S. Skokov, J. M. Bowman and K. A. Peterson, J. Chem. Phys., 1998, 109, 2662]. The calculations are carried out for energies up to 10 kcal mol-1 above the dissociation limit with a truncation/recoupling method and a Chebyshev propagation/filter diagonalization. Final rotational states distribution of the OH fragment are also reported for two contrasting resonances, using the truncation/recoupling method.

Ab initio based potential energy surfaces, microwave spectrum, and scattering cross section of the ground state Ne–Cl2 system

The high-level ab initio potential energy surface (PES) for NeCl2 in the ground electronic state predicts the energy minimum in the linear geometry (L-well) to be slightly deeper than that in the T-shaped geometry (T-well). The experimental D0 and R0 values are reproduced within uncertainties of measurements by both adding the calculated perturbation of the Ne–Cl interactions due to intramolecular forces in Cl2 to empirical NeCl potentials, and by linearly extrapolating or simply scaling the ab initio PES. These procedures lead to equal or even reversed relative depths of the two wells, in accord with both predictions of an atom-atom model using equivalently accurate ab initio NeCl potentials and variation of the ab initio PES with increasing accuracy of calculations. The D0 value for the L-well is predicted to be less than that for the T-well by 2.4 to 5.2 cm−1 for different scaling schemes. The calculated lowest energy rovibrational states associated with each of two conformers show negligible mutual influence, while the effect of the L-well on the rovibrational wave functions for the next vibrational states associated with the T-well is found to be rather important. Microwave spectra are predicted for each PES obtained, and include portions originating from the L-well. The calculated scattering cross section reproduces well the experimental data and is found to be significantly contributed by the L-well.

Quantum mechanical calculation of line shape parameters for the depolarized Raman Q branch of D2 in He

Results of quantum mechanical calculations of the spectral line widths, shifts, and line mixing parameters for the depolarized Ramanspectrum of D2 infinitely diluted in He are presented. An accurate ab initio H2-He potential was used for the converged close-coupling calculations. The predicted trends for the D2-He mixture agree reasonably well with measurements in pure D2.

Test of accuracy of diatomic potential energy function calculated by inversion of spectroscopic data

The method of direct numerical inversion of a spectrum to the generalized potential energy function (GPEF) with correct long-range part is proposed. The reliability of the method was tested by its application to the X 1 Σ + state of HeH +: the GPEF was calculated from vibrational-rotational transitions belonging to the fundamental v = 1 ← 0 band. The obtained GPEF reproduces the wavenumbers of transitions used in the fit with experimental accuracy (0.002 cm −1) and predicts the wavenumbers of transitions belonging to the v = 2 ← 1 and v = 3 ← 2 bands with a mean absolute error of 0.065 cm −1. The maximum difference between the GPEF and accurate ab initio potential of HeH + in the attractive branch is 30 cm −1 (0.2% of dissociation energy). The advantages of the proposed method are discussed.

The mobility of potassium ions in helium

The mobility of ions in helium has been measured at 76.8 K and E/N values up to 15 Td, and at room temperature up to 300 Td. The experimental scatter is of the order of 0.1 - 0.2%. The experimental mobilities are compared with values calculated from a new and accurate ab initio potential (Røeggen et al 1996). The agreement between experimental and theoretical values is significantly better than in earlier studies of this ion - atom system, but some unexplained discrepancies of the order of 1% still exist.

Coupled channel bound states calculations for alkali dimers using the Fourier grid method

The Fourier grid Hamiltonian method is shown to be a powerful method to provide an accurate determination of the bound state spectra of coupled electronic states in alkali dimers. Using accurate ab initio potentials, the perturbations in the spectra of the coupled states A 1Σ+u and b 3Πu in Na2 are reproduced in excellent agreement with spectroscopic studies. A few predictions are also presented for the heavier species Cs2, for which a complete study, both experimental and theoretical, is still needed.