The interaction of two electronic systems at short and medium range

Abstract

In the quantum theory of intermolecular forces, emphasis usually falls on long-range effects such as the dispersion interactions. At shorter range the electron distributions begin to overlap appreciably, serious non-orthogonality problems arise, and the nature of the interactions is less well understood. This paper is devoted to a general analysis of the short-range inter­actions between any pair of electronic systems, A and B, in states of arbitrary spin multi­plicity and described by wavefunctions of arbitrarily high accuracy. The interaction energy is developed as a series of terms associated with interchange of 0, 1, 2, . . . electrons (arising from the use of a fully antisymmetric wave function): the final expression is written as a power series in the expectation value of the spin scalar product S A ·S B , with numerical coefficients which are determined by the electron density, spin density, etc., of the separate systems. Numerical results are presented for two neon atoms (the closed-shell case, with spins S A = S B = 0) and for two nitrogen atoms (open shells with S A = S B = 3/2). The results, obtained from the use of one-determinant wavefunctions for the separate systems, are in good agreement with the best results so far available.