Calculation of thermodynamic properties of vapor–liquid equilibria using ab initio intermolecular potential energy surfaces for dimer O2–O2

The two 5-site potentials from ab initio calculations at the theoretical level CCSD(T) with correlation consistent basis sets aug-cc-pVmZ (with m?=?4, 34) have been constructed from oxygen. The extrapolation ab initio energies were approximated by the basis sets aug-cc-pVmZ (m?=?3, 4). These two potentials were constructed by using the ab initio intermolecular energy values and a non-linear least-squares fitting method. The second virial coefficients of oxygen were determined to demonstrate the accuracy of these ab initio 5-site potentials. These ab initio potentials were employed to estimate the thermodynamic properties of the vapor–liquid equilibria by GEMC simulation. The influence of ab initio potential alone and plus 3-body interaction Axilrod-Teller potential was investigated within GEMC simulation from 80?K to 140?K. The discrepancy between them is insignificant. This showed that the two 2-body 5-site potential functions can also be used together with the 3-body interaction Axilrod-Teller potential to generate the accurate thermodynamic properties of the liquid–vapor equilibria.     

Monte Carlo simulations of nitrogen using an ab initio potential

A new ab initio pair potential for nitrogen has been calculated at CCSD(T) level with aug-cc-pVDZ and -pVTZ correlation consistent basis sets. The results were extrapolated to approximate the basis set limit. This potential was used within Gibbs ensemble Monte Carlo (GEMC) simulations to obtain the densities of the coexisting phases, the vapour pressure and the enthalpy of vaporization from 70 K to close to the critical point. The influence of several 3-body interactions (an approximate anisotropic triple dipole potential derived by Stogryn, the isotropic triple dipole potential by Axilrod and Teller (AT), and a 3-body induction potential on the above mentioned properties was investigated. Satisfactory agreement with experimental data was observed. To determine whether the remaining deviations between experimental and computed data are due to inaccuracies in the 2-body or 3-body potential, the 2-body potential was rescaled to reproduce experimental 2nd virial coefficients accurately, and some of the calculations were repeated with the new potential. It turns out that an accurate 2-body potential only in connection with the AT potential yields accurate results for the thermodynamic properties phase equilibria.     

Ab initio intermolecular potentials of methane,nitrogen and methane + nitrogen and their use in Monte Carlo simulations of fluids and fluid mixtures

Intermolecular pair potentials of methane and of methane + nitrogen have been calculated by quantum chemical ab initio methods. The repulsive and electrostatic parts were determined pointwise for various distances and orientations of the dimers (supermolecule approach) by self-consistent field (SCF) calculations including the counterpoise correction. Gaussian basis functions of triple-zeta quality plus one set of polarization functions on all atoms were used. The dispersion energy, which cannot be calculated at the SCF level, has been added by a semi-empirical estimate. These potentials and the pair potential for nitrogen (of similar quality, taken from the literature) were fitted to analytical functions and used for NVT Monte Carlo simulations of thermodynamic properties of the fluids and their mixture over a wide temperature/density area. Comparison with measurements and with Monte Carlo results from the literature (pressure, internal energy, radial distribution function) obtained with other pair potentials indicates the quality of the present calculations.     

Electrical resistivity of NaPb compound-forming liquid alloy using abinitio pseudopotentials

The study of electrical resistivity of compound-forming liquid alloy, NaPb, is presented as a function of concentration. Hard sphere diameters of Na and Pb are obtained through the interionic pair potentials evaluated using Troullier and Martinsab initio pseudopotential, which have been used to calculate the partial structure factors S(q). Considering the liquid alloy to be a ternary mixture, Ziman formula, modified for complex formation has been used for calculating resistivity of binary liquid alloys. Form factors are calculated usingab initio pseudopotentials. The results suggest that Ziman formalism, when used withab initio pseudopotentials, are quite successful in explaining the electrical resistivity data of compound-forming binary liquid alloys.     

Ab initio potential energy curve for the neon atom pair and thermophysical properties of the dilute neon gas. I. Neon–neon interatomic potential and rovibrational spectra

A neon–neon interatomic potential energy curve was derived from quantum-mechanical ab initio calculations using basis sets of up to t-aug-cc-pV6Z quality supplemented with bond functions and ab initio methods up to CCSDT(Q). In addition, corrections for relativistic effects were determined. An analytical potential function was fitted to the ab initio values and utilised to calculate the rovibrational spectra. The quality of the interatomic potential function was tested by comparison of the calculated spectra with experimental ones and those derived from other potentials of the literature. In a following paper the new interatomic potential is applied in the framework of the quantum-statistical mechanics and of the corresponding kinetic theory to determine selected thermophysical properties of neon governed by two-body and three-body interactions.     

Ab initio pair potential energy curve for the argon atom pair and thermophysical properties of the dilute argon gas. I. Argon–argon interatomic potential and rovibrational spectra

An argon–argon interatomic potential energy curve was derived from quantum-mechanical ab initio calculations using basis sets of up to d-aug-cc-pV(6+d)Z quality supplemented with bond functions and ab initio methods up to CCSDT(Q). In addition, corrections for relativistic effects were determined. An analytical potential function was fitted to the ab initio values and utilised to compute the rovibrational spectrum. The quality of the interatomic potential function was tested by comparison of the calculated spectrum with experimental ones and those derived from other potentials of the literature. In a following paper the new interatomic potential is used to determine selected thermophysical properties of argon by means of quantum-statistical mechanics and the corresponding kinetic theory considering two-body and three-body interactions.     

Ab initio pair potential and phase equilibria predictions for the refrigerant methyl fluoride

Ab initio pair interaction energies for methyl fluoride (CH₃F) were calculated using symmetry adapted perturbation theory with the aug-cc-pVDZ basis set plus bond functions for a large number of configurations of a pair of methyl fluoride molecules in order to obtain a good representation of the two-body potential energy surface. These interaction energies were used to develop a site-site intermolecular potential that accurately reproduced the calculated energies and had the correct asymptotic behaviour at long range. This pair potential was used in Gibbs ensemble Monte Carlo simulations to predict the phase behaviour of methyl fluoride. The predicted equilibrium properties with the ab initio pair potential are in good agreement with the experimentally measured phase boundary, and give an estimated critical point in excellent agreement with measured values. Multi-body effects were then added to the pair potential using a polarizable model. The small changes to the phase behaviour that resulted established that pairwise interactions account for most of the molecular interactions in CH₃F.     

An accurate,global, ab initio potential energy surface for the H+ 3 molecule

A new global, ground-state, Born-Oppenheimer surface is presented for the H⁺ ₃ system. The energy switching approach has been used to combine different functional forms for three different regimes: a spectroscopic expansion at low energy, a Sorbie-Murrell function at high energy and known long-range terms combined with accurate diatomic potentials at large separations. At low energies we have used the ultra high accuracy ab initio data of Cencek et al. (1998, J. chem. Phys., 108, 2831). At intermediate energy we have calculated 134 new ab initio energies using a high accuracy, explicitly correlated procedure. The ab initio data of Schinke et al. (1980, J. chem. Phys., 72, 3909) has been used to constrain the high energy region. Two fits are presented which differ somewhat in their behaviour at energies over 45 000 cm^?1 above the H⁺ ₃ minimum. Below this energy, the fits reproduce each set of ab initio data close to their intrinsic accuracy. The ground state surface should provide a suitable starting point for renewed studies of the near-threshold photodissociation spectrum originally reported by Carrington et al. (1982, Molec. Phys., 45, 753).     

Three-body effects in hydrogen fluoride: survey of potential energy surfaces

The potential energy surface for trimers of hydrogen fluoride is examined for multiple arrangements of the three-molecule cluster. Several established approaches to model the potential energy are examined, including a strictly pairwise additive potential, an established polarizable potential model, another, strictly three-body polarizable model, and a three-body potential recently fitted to accurate ab initio calculations. These potential surfaces are compared to MP2/6-311++G** and SCF/6-311++G**ab initio calculations performed here for each configuration. In each case the overall trimer potential is examined, as well as the three-body contribution to it (obtained by subtracting the sum of the interactions taken pairwise). The effective pair potential has some correspondence to the ab initio calculations, although it generally displays a shallower minimum energy. The established polarizable model has a more repulsive core that compensates for a deeper attractive well that it has adopted to better describe phase-coexistence data. In contrast, the new three-body polarizable model shows better correspondence with the ab initio potential-energy surface.     

Improved techniques for the calculation of ab initio ion-neutral interaction potentials: application to coinage metal ions interacting with rare gas atoms

Ab initio values for the potential energy functions for ion–neutral interactions can be tested by comparison with gaseous ion transport coefficients, but only if special care is taken to compute the interaction potentials accurately over wide ranges of internuclear separation. This is illustrated here by a reanalysis of the ab initio values for the coinage metal ions interacting with rare gas atoms, precise calculations of the transport cross sections over extremely wide ranges of energy, and similarly precise calculations of the zero-field ion mobilities as functions of gas temperature and the field-dependent ion mobilities at various fixed temperatures. The calculations indicate that the mobilities for Ag⁺(¹S) moving in Ne or Ar can distinguish between the existing, very similar ab initio potentials. They also show that substantial differences exist among the mobilities of the coinage metal anions and the ground and excited states of the cations. The techniques implemented are recommended for future ab initio calculations.     

Nuclear quantum effects in liquid water from path-integral simulations using an ab initio force-matching approach

We have applied path integral simulations, in combination with new ab initio based water potentials, to investigate nuclear quantum effects in liquid water. Because direct ab initio path integral simulations are computationally expensive, a flexible water model is parameterised by force-matching to density functional theory-based molecular dynamics simulations. Static and dynamic properties of liquid water at ambient conditions are presented and the role of nuclear quantum effects, exchange-correlation functionals and dispersion corrections are discussed in regards to reproducing the experimental properties of liquid water.     

Is the Ar—Br2(X1Σ+ g ) van der Waals complex linear rather than T-shaped? A study in terms of ab initio based potential energy surfaces

The ground state Ar—Br₂ potential energy surface is predicted from ab initio calculations and from an atom—atom model using empirical ArBr potentials and the (evaluated ab initio) perturbation of the interaction between Ar and Br within Br₂. At all levels of modelling, the surface has a double-minimum topology, with wells for both the linear (L-) and T-shaped geometries. This differs from the single-minimum topology predicted by the commonly used pairwise additive Lennard-Jones potential. For both ab initio and atom—atom model surfaces, the L well is found to be significantly deeper than the T well; this relative behaviour is unchanged by zero-point vibrations. Spectroscopic parameters are predicted for the present surfaces. The final surfaces result from a scaling to reproduce the estimated bond energy of the system. Possible reflections of the surface topology in experimental observables are discussed.     

Spectroscopy and dissociation of sulfuryl halides SO2X2 (X=F,Cl)

The spectroscopy and dissociation of the sulfuryl halides SO₂F₂ and SO₂Cl₂ have been studied in detail using ab initio methods. The possibility of various dissociation channels has been explored taking into account that the fragmented atoms and molecules can stay in their ground state only. An interesting pattern was observed in their dissociation energy spectra for the dissociation channels. The singlet potential energy surfaces for the exit channels of these molecules have been analysed. The release of halogen molecules after dissociation is discussed from an industrial point of view. Finally, the enthalpy of formation of these molecules was computed using the ab initio results. Our results agree very well with the experimental values available.     

Interaction potentials,spectroscopy and transport properties of RG+–He (RG=Ar–Rn)

High-level ab initio potential energy curves are calculated for the RG⁺–He complexes (RG=Ar–Rn). RCCSD(T) calculations are employed with large basis sets, and taking account of spin–orbit coupling. The calculated spectroscopic parameters are compared with experimentally determined values, with other high-level ab initio results, and with results from potentials that were obtained by fitting to experimental data. The gas-phase mobilities of RG⁺ ions in He are calculated from our potentials and compared, graphically and statistically, with the experimental mobilities as a function of E/n ₀ at several temperatures. We conclude that more precise experimental data are required in order to discriminate between potentials with more certainty. In addition, we discuss previously reported, unexpectedly large drops in experimental mobility values for RG⁺ in He at 4.35 K as E/n ₀ → 0.     

Structural and response functions at equilibrium in path-integral quantum simple fluids

A discussion on the physical meaning of the r-space structures that can be defined in path-integral quantum simple fluids far from exchange is presented by making the connection with their associated experimentally measurable properties in k-space (response functions). The role played in this issue by weak external fields acting on the fluid is examined by considering both the standard quantum treatment of neutron scattering and the path-integral functional analysis approach. For the sake of completeness, the same discussion is presented for the approximate Gaussian Feynman-Hibbs effective potential picture that can be derived from the path-integral, and also the structural interrelations between both formalisms are stated. To illustrate the points addressed in this paper results for liquid helium-4 at 4.2 K (SVP), obtained with the use of the Aziz-Slaman and the ab initio SAPT2 pair potentials, are reported.     

Ab initio potential energy curve for the helium atom pair and thermophysical properties of dilute helium gas. I. Helium–helium interatomic potential

A helium–helium interatomic potential energy curve was determined from quantum-mechanical ab initio calculations. Very large atom-centred basis sets including a newly developed d-aug-cc-pV8Z basis set supplemented with bond functions and ab initio methods up to full CI were applied. The aug-cc-pV7Z basis set of Gdanitz (J. Chem. Phys. 113, 5145 (2000)) was modified to be more consistent with the aug-cc-pV5Z and aug-cc-pV6Z basis sets. The diagonal Born–Oppenheimer corrections as well as corrections for relativistic effects were also calculated. A new analytical representation of the interatomic potential energy was fitted to the ab initio calculated values. In a following paper this potential model will be used in the framework of quantum-statistical mechanics and of the corresponding kinetic theory to calculate the most important thermophysical properties of helium governed by two-body and three-body interactions.     

Transport properties of He-CO mixtures

Classical trajectory calculations for the collision of He with CO have been carried out, with CO treated as a rigid rotor, for two similar potential energy surfaces, one obtained from high level ab initio calculations and the other from a semi-empirical fit to the infrared spectra of He-CO van der Waals dimers. Second-approximation corrections to the kinetic-theory expressions have been employed in order to carry out comparisons with the most accurate and precise diffusion, shear viscosity, thermal conductivity and thermal diffusion data available. Rotational relaxation data are also considered. Both potentials are found to have good predictive power for these transport and relaxation properties, giving on the whole very good agreement with the experimental data. This indicates that the isotropic components of the two potential surfaces are very similar, and that the anisotropies are not significantly different. It is suggested that the measurement of additional relaxation phenomena in He-CO mixtures could allow a clearer distinction to be made between the two potential surfaces.     

Accurate second Kerr virial coefficient of rare gases from the state-of-the-art ab initio potentials and (hyper)polarizabilities

The second Kerr virial coefficient of rare gases is studied in this work using the best ab initio potentials and (hyper)polarizabilities in the literature. The second Kerr virial coefficient of helium-4, helium-3, neon, argon, and krypton and its polarizability component of xenon are computed by the semi-classical method together with the Padé approximant over a wide temperature range. In addition, the uncertainty of second Kerr virial coefficient is estimated from the uncertainties of the ab initio interaction-induced properties. The experimental and theoretical data in the literature is compared with our calculated values to examine the quality of this work. It is shown that our computed values in the supplementary materials are as accurate as the literature data at medium and high temperatures and are more reliable at low temperatures.     

高次激发相关能在构建HCN-He亚光谱精度势能面中的角色:通过实验高分辨率光谱进行严格检测

Interpreting high-resolution rovibrational spectra of weakly bound complexes commonly requires spectroscopic accuracy (<1 cm^-1) potential energy surfaces (PES). Constructing high-accuracy ab initio PES relies on the high-level electronic structure approaches and the accurate physical models to represent the potentials. The coupled cluster approaches including single and double excitations with a perturbational estimate of triple excitations (CCSD(T)) have been termed the "gold standard" of electronic structure theory, and widely used in generating intermolecular interaction energies for most van der Waals complexes. However, for HCN-He complex, the observed millimeter-wave spectroscopy with high-excited resonance states has not been assigned and interpreted even on the ab initio PES computed at CCSD(T) level of theory with the complete basis set (CBS) limit. In this work, an effective three-dimensional ab initio PES for HCN-He, which explicitly incorporates dependence on the Q₁ (C-H) normal-mode coordinate of the HCN monomer has been calculated at the CCSD(T)/CBS level. The post-CCSD(T) interaction energy has been examined and included in our PES. Analytic two-dimensional PESs are obtained by least-squares fitting vibrationally averaged interaction energies for v₁(C-H)=0, and 1 to the Morse/Long-Range potential function form with root-mean-square deviations (RMSD) smaller than 0.011 cm^-1. The role and significance of the post-CCSD(T) interaction energy contribution are clearly illustrated by comparison with the predicted rovibrational energy levels. With or without post-CCSD(T) corrections, the value of dissociation limit (D₀) is 8.919 or 9.403 cm^-1, respectively. The predicted millimeter-wave transitions and intensities from the PES with post-CCSD(T) excitation corrections are in good agreement with the available experimental data with RMS discrepancy of 0.072 cm^-1. Moreover, the infrared spectrum for HCN-He complex is predicted for the first time. These results will serve as a good starting point and provide reliable guidance for future infrared studies of HCN doped in (He)_n clusters.