Anisotropic dispersion forces in methane mixtures

Abstract

Isotropic and anisotropic dispersion coefficients C ₆, C ₇ and C ₈ are obtained for CH₄-CH₄, CH₄-He, CH₄-Ne and CH₄-Ar pairs within the ab initio random-phase approximation (equivalent to coupled Hartree-Fock theory) by calculating frequency-dependent dipole, quadrupole, mixed dipolequadrupole and mixed dipole-octopole polarizabilities. The ab initio results are compared with Kirkwood and bond polarizability models and used to parameterise Padé approximants for α, A and E polarizabilities of methane. The near-Hartree-Fock wave-function for CH₄ recovers 93 per cent of the experimental dipole polarizability and corresponding fractions of the empirical dispersion coefficients.