A b i n i t i o calculations of the collision-induced dipole in He–H2. I. A valence bond approach

Abstract

The collision‐induced dipole in the system He–H₂ is calculated in the multistructure Valence Bond method, using the nonorthogonal monomer orbitals. In the region around the collision diameter, which contributes most to the collision‐induced ir absorption, the long range results (the leading terms are the quadrupole‐induced dipole on He with R⁻⁴ dependence and the dispersion dipole with R⁻⁷ dependence) are modified by overlap effects. The short range behavior is determined, moreover, by the appearance of other important terms, the exchange dipole and the overlap‐induction dipole on H₂, which vanish in the long range. Since all the short range contributions have approximately the same (exponential) dependence on the intemolecular distance, they can be collected and added as a single exponential dipole function to the R⁻⁴ and R⁻⁷ long range terms. Of the latter terms the R⁻⁷ dispersion dipole is of little importance.