Anisotropic dispersion forces in methane mixtures

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.