D9 dipole dispersion coefficients

Abstract

A formula is derived for D 9, the first correction to the usual single-term approximation for the dispersion dipole of a pair of dissimilar atoms. The expression is a Casimir—Polder integral involving dipole and quadrupole polarisabilities, dipole—dipole—quadrupole and dipole— quadrupole—octopole hyperpolarisabilities on each atom. A coupled Hartree—Fock scheme for its ab initio calculation is implemented and static approximation to D 9 show it to be of the same sign as D 7 but larger in magnitude (when both are expressed in atomic units). CHF calculations on H and He give D 9(H +He −)=−3390 au, i.e. over 25 a 2 0 times D 7, and the R −9 local dipole in H…H is over 30 R −2 times the better known R −7 moment.