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.     

An intermolecular pair potential energy function for methane

A numerical spherically averaged intermolecular pair potential energy function is presented which describes the interactions of methane molecules. The function incorporates accurate estimates of the long-range dispersion force coefficients, and closely reproduces shear viscosity and second virial coefficient data over a wide temperature range. Agreement with other experimental data which is generally satisfactory is also discussed. The potential is shown to be superior to a large number of analytical potentials previously proposed for this gas.     

Comment on the determination of the intermolecular potential energy function of neon from spectroscopic,equilibrium and transport data

A quantum-mechanical treatment of the dynamics of relaxation of a twolevel mode coupled to a thermal bath is presented. The Zwanzig-Mori projection-operator formalism is employed to derive exact equations of motion for the correlation function and the excess population , where and are the coordinate and number operators associated with the two-level mode. In the van Hove weak-coupling limit it is shown that |G _ν|² and Δn decay exponentially with time constants τ and τ′, respectively. Explicit expressions for τ and τ′ are determined and the relationship between them clearly established. The application of the model to the specific problem of the effects of vibrational relaxation on isotropic Raman spectral lineshapes in dense media is emphasized.     

Light scattering from gaseous nitrogen: Effects of correlation between molecular and intermolecular axis

The depolarization ratio of light scattered from gaseous nitrogen has been measured at pressures up to 440 bar and 310 K. The molecular specific scattering (including the effects of the orientational pair correlations labelled OPC), the collision induced scattering (CIS), and their interferences are estimated in the DID approximation. The agreement with the experimental data is rather good.     

Anisotropy of the repulsive intermolecular potential from rotationally inelastic scattering

Structure is observed in recoil velocity distributions of potassium atoms scattered inelastically from N₂ and CO molecules at CMS angles?>π/2 and collision energies 0.34≦E≦1.24 eV. It is caused by rotational excitation mainly. Individual rotational transitions are not resolved. The quasi-continuous recoil velocity distributions extend between well defined bounds, the upper corresponding to elastic scattering, the lower marking a largest amount of energy transferred into molecular rotation at given? andE. This maximal transfer increases with angle and — forE?0.64 eV — in near proportion toE. At all? andE it is larger for CO than forN ₂, amounting to about 0.42E forN ₂ and 0.64E for CO at?=150°. At or very close to each of their bounds the distributions exhibit pronounced maxima. A third intermediate maximum is present for CO, but missing for N₂. The scattering in these experiments is dominated by the repulsive core of the interaction potential. On the basis of classical scattering from a rigid, ellipsoidal, initially nonrotating potential shell, the observations can be qualitatively understood as follows: The recoil velocity dependence of the orientation averaged cross section at fixed observation angle will exhibit classical, integrable singularities at extremal amounts of rotational energy transfer. If the center of symmetry of the shell coincides with the center of mass of the molecule, there will be two such extremal transfers,ΔE=0 and a largest transfer possible at the chosen angle. If the centers do not coincide, the singularity at the largest transfer will split into two, one corresponding to the short, the other to the long “lever arm” of the shell. K + N₂ is an example of the centrical, K+CO of the excentrical case. The finite energy losses, at which the singularities occur, are — at each angle — strongly dependent on the anisotropy and excentricity of the shell.     

Anisotropic intermolecular potential from nuclear spin-lattice relaxation in hexafluoride gases

The data on fluorine spin-lattice relaxation times per unit densityT _jσ in pure SF₆ and UF₆ gases can be analyzed to obtain information on the anisotropic part of the intermolecular potential in these systems. A new and more performant potential, Morse-Morse-Spline-van der Waals potential (J. Chem. Phys.94, 1034 (1991)) was used for the isotropic part of the intermolecular interaction. The analysis was made using the Bloom-Oppenheim theory, assuming, that the correlation time of the spin-rotation interaction can be approximated by the average lifetime of a molecule in a givenJ state. We have obtained the strengths of the repulsive and attractive terms in the anisotropic potential. From the strength of the attractive term, the hexadecapole moment of SF6 and UF6 were also obtained, being in good agreement with the values reported earlier, based on other potentials and techniques.     

Comment on: Bag like potential and quarkonium

Some errors in a recent article by Zadoo and Sofi are discussed.     

Raman spectra of fluid nitrogen: intermolecular torques and orientational correlation times

A comparison is made between the rotovibrational Raman bands of fluid nitrogen at room temperature in the 180–355 amagat density range and the corresponding pure rotational bands reported previously. Accurate moment values are derived from the Raman spectra, accounting for the response of the entire experimental apparatus, after subtraction of the unwanted isotope band and of the leaking polarized Q branch. Correlation times are also calculated for both rotovibrational and pure rotational spectra accounting for the finite instrumental slitwidth.

Intermolecular torques and orientational correlation times derived from the two spectra are in good agreement for almost the entire available density range. Small discrepancies, particularly evident in the low density correlation times, are discussed and related to the rotation-vibration interaction. Intermolecular correlations are found to be negligible.     

He-HF体系各向异性势及分波散射截面的研究

用BFW势函数拟合在CCSD(T)/aug-cc-pVQZ理论水平下计算的He-HF相互作用能数据,获得了He原子与HF分子相互作用的各向异性势,并与已知的势模型进行比较,验证了拟合势的可靠性;然后采用密耦方法计算了He-HF体系在不同碰撞能量下的分波散射截面,总结了分波散射截面的变化趋势.研究表明:①拟合势不但表达形式简洁,而且较好地描述了He-HF系统相互作用的各向异性特征.②入射粒子的能量越高,得到收敛的分波截面所需的分波数越多,量子效应越不显著,尾部效应也越弱;尾部效应仅在低激发态中产生,高激发态不产生尾部效应.     

Revised intermolecular potential with parameters depending on partial atomic charges for aromatic molecular systems

The practicablity and precision of the intermolecular potential was improved within the frame of an atom-atom approximation using 6-exp-1 type isotropic interatomic potentials without additional parameters. The improvement involves making the van der Waals parameters functions of the partial charges on constituent atoms supplement the classification of the parameter set, depending on atom type. The validity of the potential was confirmed with respect to crystal structures of polyacene (i.e., benzene, naphthalene, anthracene and tetracene) by molecular dynamics simulations.     

Numerical solution of the linearized Boltzmann equation for an arbitrary intermolecular potential

A numerical procedure to solve the linearized Boltzmann equation with an arbitrary intermolecular potential by the discrete velocity method is elaborated. The equation is written in terms of the kernel, which contains the differential cross section and represents a singularity. As an example, the Lennard-Jones potential is used and the corresponding differential cross section is calculated and tabulated. Then, the kernel is calculated so that to overcome its singularity. Once, the kernel is known and stored it can be used for many kinds of gas flows. In order to test the method, the transport coefficients, i.e. thermal conductivity and viscosity for all noble gases, are calculated and compared with those obtained by the variational method using the Sonine polynomials expansion. The fine agreement between the results obtained by the two different methods shows the feasibility of application of the proposed technique to calculate rarefied gas flows over the whole range of the Knudsen number.     

The intermolecular potential and its angular dependence for two H2 molecules

The potential energy surface in the non-reactive region has been determined theoretically for the H₂-H₂ system. The procedure consists of a non-orthogonal configuration interaction calculation using the individual SCF orbitals of the separate molecules. The potential function is expressed as a 5-term sum of Legendre functions, and analytical expressions are given for the R dependence of the terms. The calculated depth of the spherically averaged Van der Waals well is -2·96 meV, which is in essentially complete agreement with the experimental value of -3·00 meV. the position of the minimum is at 3·49 Å both theoretically and experimentally. The value of C ₆ for dispersion forces obtained in this calculation is 12·97 a.u.