Anisotropy of long range interactions between linear molecules: H2-H2 and H2-He

We derive a closed expression for the orientation dependence of the long range interaction coefficients of Σ state linear molecules and then calculate the dispersion and induction multipole interaction coefficients for the systems H2-H2 and H2-He up to R -10 terms inclusive. The monomer states are described by SCF-LCAO wave functions with polarization functions optimized with respect to the different multipole polarizabilities. The anisotropy factors γ8 and γ10, describing the orientation dependence of the R -8 and R -10 terms, are approximately equal and much larger than γ6, due to the occurrence of the (completely anisotropic) mixed-pole terms.