Abstract
Calculations on He+2 have been carried out with atom-optimized valence bond functions. These are compared with multiconfiguration VB and with MO calculations. For R > 3 a.u. the VB curves are nearly parallel, showing that the binding energy can be interpreted by first-order hole-exchange energies. The MO function, which has the wrong dissociation limit, has a higher energy than the simplest VB function for R > 2 a.u. showing that the contraction energy from a localized positive charge is appreciably more than that from two half charges even when hole exchange is important. By comparing an inner-shell ionized N+2 with He+2 at large R(such that the 1s overlap integrals are comparable) we conclude that the inner shells in N+2 are best represented by a VB function.
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