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

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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.