ONE-CENTER MULTIPOLE CALCULATION OF 2ND-ORDER PERTURBATION ENERGIES FOR H2+

A knowledge of the H(1s) pseudospectrum allows one to calculate the on e-centre second-order perturbation energies for the H(1s)-H+ interacti on. The efficiency of such calculations is tested in detail for induct ion, exchange-induction, first-order overlap and Murrell-Shaw-Musher-A mos second-order energies for the ground and excited state H2+ at diff erent internuclear distances. It is found that a large number of STO f unctions accounting for at least the first sixteen non-expanded multip oles is needed to obtain three figure accuracy in the short to medium range of internuclear separations. Higher accuracy can be obtained onl y in terms of a sensibly smaller number of two-centre functions which automatically satisfy the electron-nucleus cusp requirement.