The construction of modified virtual orbitals (MVO’s) which are suited for configuration interaction calculations

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Abstract
A simple method, requiring the work of a single SCF iteration, is presented for the construction of modified virtual orbitals (MVO’s). Test calculations on H2O, Ar, and two states of CH2 show that for the truncation of orbitals at the CI level, the MVO’s are far more efficient than canonical SCF virtual orbitals or such related SCF virtuals as improved virtual orbitals (IVO’s) and for some cases approach the efficiency of natural orbitals. MVO’s are shown to be suitable for configuration selection based on energy contributions. The CI wave function is observed to be more compact with MVO’s, allowing better interpretation of the results.