Collision-induced absorption (CIA) spectra and ground-state potentials of inert gas mixtures

Collision-induced absorption spectra of the rare gas systems He-Ne, He-Ar, He-Kr, He-Xe, Ne-Kr, Ne-Xe, Ar-Kr and Ar-Xe at different temperatures with the pressure second virial coefficients, viscosity and thermal conductivity have been used for deriving the empirical models of the induced dipole moment and the interaction potential. Theoretical zeroth, first and second moments of the binary spectra using various models for the induced dipole moment and interatomic potential are compared with the experimental values performed by the groups of Marteau, Bosomworth, Bucktoyarova, Bar-Ziv, Ryzhov and Frommhold. In addition, mixture diffusion coefficients and isotopic thermal factors calculated for these models are compared with experimental ones. The results show that these models are the most accurate models reported to date for these mixtures.     

An approximate multi-property empirical isotropic interatomic pair potential for the krypton dimer

An approximate empirical isotropic interatomic potential for krypton interaction is developed by simultaneously fitting the Morse-Morse-Morse-Spline-van der Waals potential form to the pressure second virial coefficient, viscosity, thermal conductivity and depolarized interaction-induced light scattering data. Absolute zeroth and second moments of the two-and three-body spectra, the pressure third virial coefficient and isotopic thermal diffusion factor have been measured and compared with theoretical calculations using various models for the interatomic potential. The results show that it is the most accurate potential yet reported for this system. The use of the new potential in lattice sum calculations yields good results for several properties of solid krypton.     

Ab initio potential energy surface for the nitrogen molecule pair and thermophysical properties of nitrogen gas

A four-dimensional potential energy hypersurface (PES) for the interaction of two rigid nitrogen molecules was determined from high-level quantum-chemical ab initio computations. A total of 408 points for 26 distinct angular configurations were calculated utilizing the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory and basis sets up to aug-cc-pV5Z supplemented with bond functions. The calculated interaction energies were extrapolated to the complete basis set limit and complemented by corrections for core–core and core–valence correlations, relativistic effects and higher coupled-cluster levels up to CCSDT(Q). An analytical site–site potential function with five sites per nitrogen molecule was fitted to the interaction energies. The PES was validated by computing second and third pressure virial coefficients as well as shear viscosity and thermal conductivity in the dilute-gas limit. An improved PES was obtained by scaling the CCSDT(Q) corrections for all 408 points by a constant factor, leading to quantitative agreement with the most accurate experimental values of the second virial coefficient over a wide temperature range. The comparison with the best experimental data for shear viscosity shows that the values computed with the improved PES are too low by about 0.3% between 300 and 700?K. For thermal conductivity large systematic deviations are found above 500?K between the calculated values and most of the experimental data.     

A reliable single parameter interatomic potential for argon

A simple semiempirical approximation previously proposed for the isotropic intermolecular forces between two closed shell systems is tested in detail for the argon-argon interaction. The potential is based on the knowledge of the first-order coulomb interaction energy, a suitably damped three term long range asymptotic expansion of the second order coulomb energy, and a semiempirical representation of the exchange interaction energy which contains one adjustable parameter. The single adjustable parameter can be reliably determined by fitting the second virial coefficient for argon in the 130–773 K temperature range with the long range interaction coefficients being constrained within the theoretical bounds specified by Tang, Norbeck and Certain. The reliability of the potential is compared with that of several literature potentials by comparing the theoretical predictions obtained from the potentials with experimental results for the second virial coefficient, viscosity, thermal conductivity and thermal diffusion ratios for dilute argon gas, and with spectroscopic data for the dimer, and with SCF calculations of the Ar-Ar potential at small interatomic separations. Our best potential predicts these properties with a precision as good as or better than other recent potentials which generally contain more adjustable parameters and/or involve more input data. The results confirm earlier work that suggested that the scheme tested is capable of yielding reliable isotropic potentials for the interaction of closed shell systems for 0·3 ? R/R_m ? ∞ where R_m is the intermolecular distance at the van der Waals minimum. The scheme appears to offer a method for obtaining reliable potentials while avoiding problems associated with optimizing many parameters with respect to fitting experimental constraints.     

Transport Coefficients in Some Stochastic Models of the Revised Enskog Equation

A stochastic model of the revised Enskog equation is considered. A choice of the smearing function suggested by the work of Leegwater is used to apply the model to the repulsive part of the Lennard-Jones potential and the inverse-power soft-sphere potential. The virial coefficients obtained from the equilibrium properties of the models are in excellent agreement with the known exact coefficients for these models. The transport coefficients for the repulsive Lennard-Jones (RLP) model are also computed and appear to be of comparable accuracy to the Enskog-theory coefficients applied directly to a hard-sphere system, although exact results for the RLP with which to make an extensive comparison are not yet available. The pressure and the transport coefficients obtained from the model (shear viscosity, thermal conductivity, and self-diffusion) are compared with the pressure and the corresponding transport coefficients predicted by the Enskog and square-well kinetic theories.     

Ab initio potential energy curve for the neon atom pair and thermophysical properties for the dilute neon gas. II. Thermophysical properties for low-density neon

A neon–neon interatomic potential energy curve determined from quantum-mechanical ab initio calculations and described with an analytical representation (R. Hellmann, E. Bich, and E. Vogel, Molec. Phys. 106, 133 (2008)) was used in the framework of the quantum-statistical mechanics and of the corresponding kinetic theory to calculate the most important thermophysical properties of neon governed by two-body and three-body interactions. The second and third pressure virial coefficients as well as the viscosity and thermal conductivity coefficients, the last two in the so-called limit of zero density, were calculated for natural Ne from 25 to 10,000 K. Comparison of the calculated viscosity and thermal conductivity with the most accurate experimental data at ambient temperature shows that these values are accurate enough to be applied as standard values for the complete temperature range of the calculations characterized by an uncertainty of about ±0.1% except at the lowest temperatures.     

Ab initio potential energy curve for the helium atom pair and thermophysical properties of the dilute helium gas. II. Thermophysical standard values for low-density helium

A helium–helium interatomic potential energy curve determined from quantum-mechanical ab initio calculations and described with an analytical representation considering relativistic retardation effects (R. Hellmann, E. Bich, and E. Vogel, Molec. Phys. (in press)) was used in the framework of the 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. The second pressure virial coefficient as well as the viscosity and thermal conductivity coefficients, the last two in the so-called limit of zero density, were calculated for ³He and ⁴He from 1 to 10 000 K and the third pressure virial coefficient for ⁴He from 20 to 10 000 K. The transport property values can be applied as standard values for the complete temperature range of the calculations characterized by an uncertainty of ±0.02% for temperatures above 15 K. This uncertainty is superior to the best experimental measurements at ambient temperature.     

Ab initio pair potential energy curve for the argon atom pair and thermophysical properties for the dilute argon gas. II. Thermophysical properties for low-density argon

A recent argon–argon interatomic potential energy curve determined from quantum-mechanical ab initio calculations and described with an analytical representation [B. Jäger, R. Hellmann, E. Bich, and E. Vogel, Mol. Phys. 107, 2181 (2009); 108, 105 (2010)] was used to calculate the most important thermophysical properties of argon governed by two-body interactions. Second pressure, acoustic, and dielectric virial coefficients as well as viscosity and thermal conductivity in the limit of zero density were computed for natural argon from 83 to 10,000?K. The calculated values for the different thermophysical properties are compared with available experimental data and values computed for other argon–argon potentials. This extensive analysis shows that the proposed potential is superior to all previous ones and that the calculated thermophysical property values are accurate enough to be applied as standard values for the complete temperature range of the calculations.     

Ab initio calculation of the refractivity and hyperpolarizability second virial coefficients of neon gas

Second virial coefficients for the density dependence of a number of electric properties are calculated for neon gas. Employing an accurate CCSD(T) potential for the Ne² van der Waals dimer and interaction-induced electric dipole polarizabilities and hyperpolarizabilities obtained from CCSD response theory, we evaluated the dielectric, refractivity, Kerr and ESHG second virial coefficients using both a semiclassical and a quantum statistical approach. The results cover a wide range of temperatures and are expected to be more reliable than the available experimental and empirical data. Quantum effects are found to be important only for temperatures below 100 K. The frequency-dependence of the refractivity virial coefficient is found to be small, but not negligible. For frequencies in the visible region it accounts for a few percent of the final results. For the ESHG virial coefficient of neon, frequency dependence is found to be very important, accounting for 20–25% of the second virial coefficient at the typical frequencies employed in experiments.     

Atomic interactions in neon and helium

The first two quantum corrections to the second virial coefficients of the Smith-Thakkar potential are calculated. Parameters for neon and helium, gases in which quantum effects are important, are then determined by fitting to semiempirical dispersion coefficients and experimental second virial coefficients. Viscosity coefficients for both gases and vibrational energy level spacings for the neon dimer are calculated as independent tests of the potentials. Overall agreement with experiment is excellent for neon and moderate for helium. The previously determined parameters for argon are found to be only very slightly perturbed by the inclusion of quantum corrections in the calculated second virial coefficients.     

Density expansion of the magnetic susceptibility

The pressure of a system may be expanded as a power series in the density, whose coefficients are the virial coefficients. In this paper, the magnetic susceptibility of a $\text{spin}\text{-}\text{1}\text{2}$ fermion system is also expanded in powers of density. This process explicitly separates the temperature and density dependence of the magnetic susceptibility. The coefficients of this series are shown to be related to certain virial coefficients. The first (previously established) and second corrections to Curie's law are explicitly expressed in terms of second and third virial coefficients. These corrections to Curie's law are small for temperatures above 4K, but become important below that temperature. The first correction has been previously measured. Given a set of second and third virial coefficients, the importance of the second correction can be calculated immediately at any density of interest.     

Interatomic interactions and the Cotton—Mouton effect for helium

We report calculations of the interaction-induced polarizability (δα^anis), magnetizability (δξ^anis;) and hypermagnetizability (δη^anis) anisotropies for the helium gas as a function of the interatomic separation. From these data we determine the virial coefficients for the Cotton—Mouton effect and the hypermagnetizability anisotropy of helium. We also find the mean polarizability and magnetizability as a function of the interatomic separation and the virial coefficients for these properties. The results for the Cotton—Mouton effect indicate that pressure affects the Cotton—Mouton constant to the same extent as it does the second hyperpolarizability (γ) and the virial coefficient b^CME(ω, T) lies in the range of ?1.6 to ?1.8 cm³ mol^?1. This means that pressure effects for the Cotton-Mouton constant could be detected with modern experimental techniques. All calculations were carried out using the full configuration interaction technique and large basis sets of London atomic orbitals. The polarizability calculations were performed both for relevant optical frequencies as well as the static case.     

H-H and C-H coupling constants in neutral and protonated azines

New measurements are reported of the density dependent depolarization ratio for argon, krypton, xenon, methane and sulphur hexafluoride, and the results are analysed to provide values for the second and third depolarization virial coefficients. The relationships between the second depolarization virial coefficient, the zeroth moment of the two-body Rayleigh spectrum and the second Kerr virial coefficient are considered, and it is shown that they now provide consistent results for the collision-induced pair polarizability anisotropy. Former inconsistencies are attributed to insufficient allowance for the effects of three-body interactions. Calculations of the second and third depolarization virial coefficients based on the DID model and using the Maitland-Smith potential are in excellent agreement with the experimental results for argon, krypton and xenon.     

Editorial

Using coupled cluster singles and doubles linear response theory and the d-aug-cc-pVTZ basis set extended with a 3s3p2d1f1g set of midbond functions, the interaction induced electric dipole polarisability surface of the CO–Ar van der Waals complex is computed. Combining this surface with accurate intermolecular potential energy and electric dipole surfaces, the pressure and dielectric second virial coefficients of the complex are calculated by a classical statistical approach. Excellent agreement with experimental results (to within the experimental error bars) is obtained for the pressure second virial coefficient over a range of temperatures. No previous experimental or theoretical investigations have been carried out for the dielectric second virial coefficient, B _ε(T), which is estimated to be about 1.9 cm⁶ mol^??1 at room temperature. This value results from a balance of terms due to the interaction induced electric dipole polarisability (predominant at high temperatures) and orientational electric dipole contributions.     

A simple effective pair potential for the molecular simulation of the thermodynamic properties of ammonia

A new optimized effective pair potential model is proposed, which is appropriate for the prediction of thermodynamic properties of fluid ammonia including vapour—liquid coexistence data. The phase behaviour is determined using a recently developed version of the Gibbs ensemble Monte Carlo method. Furthermore, liquid structure characteristics, the dielectric constant and supercritical properties are determined by Monte Carlo simulations in the isothermal—isobaric ensemble. The second virial coefficient of the pair potential model is calculated over a broad range of temperature. All properties are compared with experimental data or results of a multi-parameter equation of state for ammonia. The new model is found to yield coexistence properties and second virial coefficients in good agreement with experimental data and the results of the equation of state, respectively.     

Thermodynamics of the Stockmayer fluid in an applied field

The thermodynamic properties of the Stockmayer fluid in an applied field are studied using theory and computer simulation. Theoretical expressions for the second and third virial coefficients are obtained in terms of the dipolar coupling constant (λ, measuring the strength of dipolar interactions as compared to thermal energy) and dipole–field interaction energy (α, being proportional to the applied field strength). These expressions are tested against numerical results obtained by Mayer sampling calculations. The expression for the second virial coefficient contains terms up to λ⁴, and is found to be accurate over realistic ranges of dipole moment and temperature, and over the entire range of the applied field strength (from zero to infinity). The corresponding expression for the third virial coefficient is truncated at λ³, and is not very accurate: higher order terms are very difficult to calculate. The virial coefficients are incorporated in to a thermodynamic theory based on a logarithmic representation of the Helmholtz free energy. This theory is designed to retain the input virial coefficients, and account for some higher order terms in the sense of a resummation. The compressibility factor is obtained from the theory and compared to results from molecular dynamics simulations with a typical value λ = 1. Despite the mathematical approximations of the virial coefficients, the theory captures the effects of the applied field very well. Finally, the vapour–liquid critical parameters are determined from the theory, and compared to published simulation results; the agreement between the theory and simulations is good.     

稠密气体中的输运现象和方位势氩的粘滞率和热导率

建议用推广的恩斯柯克方程(14),讨论稠密气体中的输运现象。指出空间关联函数Y(r)可以从状态方程的维里展开求得。文中给出了计算方位势稠密气体的粘滞率和热导率的公式以及计算关联函数的方法。用得到的结果,计算了温度为173K和348K的氩的粘滞率和温度为183K和348K的氩的热导率。在高密度区,计算结果与实验测定的符合程度,较之现有理论,有明显的改进。 关键词：     

Coupled cluster calculations of the ground state potential and interaction induced electric properties of the mixed dimers of helium,neon and argon

Using augmented polarized correlation consistent basis sets extended with midbond functions, we evaluate the ground state interaction potential and the induced electric dipole polarizabilities and first and second hyperpolarizabilities of the He–Ar, Ne–Ar and He–Ne van der Waals complexes. For the calculation of the potential we resort to the coupled cluster singles and doubles (CCSD) model corrected for triple excitations, CCSD(T), whereas properties are evaluated with CCSD response theory. As a check of the quality of the potential, the rovibrational spectrum and the gas second virial coefficients are evaluated. The rovibrational spectra improve previously available theoretical results, although the dissociation energies are probably still slightly underestimated. For the gas second virial coefficients the agreement with experiment is satisfactory. The frequency dependence of the interaction (hyper)polarizabilities is analysed and a comparison with previous results on the mixed dimers and the pure gases is made.     

Transport properties of bulk water at 243–550 K: a Comparative molecular dynamics simulation study using SPC/E,TIP4P,and TIP4P/2005 water models

ABSTRACT

Molecular dynamics simulations of various water models – SPC/E (extended simple point charge), TIP4P (transferable intermolecular potential 4 points), and TIP4P/2005 – have been carried out in the canonical (NVT fixed) ensemble over the range of temperatures 243–550?K with Ewald summation. The transport properties (self-diffusion coefficients D, viscosities η, and thermal conductivities λ) of SPC/E, TIP4P, and TIP4P/2005 water were evaluated at 243–550?K and compared with experimental data. The temperature dependence of transport properties of SPC/E, TIP4P and TIP4P/2005 water was discussed to determine how reliable the models are over this temperature range.     

Magnetic anisotropies of some aromatic molecules

A common method for the estimation of uncertainties introduced by surface and impurity effects into experimental measurements of virial coefficients is described. The sign and the amplitude of the second virial coefficient response to perturbation caused by adsorption of molecules on the internal surface of the vessel have been determined. It has been shown that the magnitude of the second virial coefficient distortion depends on such competing factors as adsorption-impurity perturbation parameter, mixture composition which has been corrected taking into account this perturbation, and the nature of the impurity expressed in terms of its second virial coefficient and of the solvent-impurity cross second virial coefficient. The character of the Lennard-Jones 12–6 potential parameters perturbation, caused by the adsorption-impurity effects, is determined using second virial coefficient data inversion technique. Numerical estimates are made for nitrogen, helium, argon, xenon, their binary mixtures, and also for krypton-sulphur hexafluoride gaseous mixtures.