A comparative study of the convergence characteristics of modified virtual orbitals within an orbitally ordered CI expansion

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Abstract
A variety of unitary transformations on the canonical set of virtual orbitals yield sets of modified virtual orbitals which display qualitatively similar convergence characteristics within a limited CI expansion in which the orbitals are ordered via their interaction with the occupied SCF orbitals. The Hartree–Fock exchange operator, with neglect of those contributions arising from the occupied core orbitals, is found on energetic grounds to be the most effective operator for mixing the canonical virtual orbitals among themselves. In addition to providing considerable energy benefits, such modified virtual orbitals possess one‐electron expectation values which are similar to those of natural orbitals, derived from an extended CI calculation.