Theory of Correlated Wavefunctions. IV. A ``Configuration Interaction Plus Perturbation'' Approach

Abstract

The perturbation—variation treatment of electron correlation is generalized to include a many‐configuration initial approximation Ψ₀ which is a linear combination of all those configurations consistent with a chosen basis set of orbitals. Such a generalization enables all configurations that make a large contribution to the wavefunction to be included in Ψ₀, so that the problem of degeneracies and near‐degeneracies is removed, and the correction to Ψ₀, being essentially small, may then be determined by first‐order perturbation theory. The initial approximation may be determined using configuration‐interaction methods, which gives upper bounds to a number of states. Excited states may be treated as easily as, and together with, the ground state. It is shown that when Ψ₀ is the optimum combination of configurations consistent with a chosen orbital basis, for any state, then it is invariant under a unitary transformation of the orbitals of the basis. This allows a transformation to orbitals which optimize the contribution of the first‐order wavefunction.