A simple procedure for the improvement of CI convergence in molecular systems

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Abstract
A simple scheme for the construction of improved virtual orbitals for use in SCF+CI wave functions is described and applied to the HCN molecule. The resultant CI energy lies well below that obtained from the use of those natural orbitals constructed from the one‐electron density arising from an identical CI calculation with the canonical SCF orbitals. The method relies, for its applicability, on the use of a CI wave function containing only a limited number of nondiagonal double excitations, and should therefore have considerable potential in the application of CI methods to larger molecules, where the incorportion of all double excitations into the wave function is unrealistic.

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