The efficiency of modified virtual orbitals (MVO) of ionic type and of approximate orthogonalized natural orbitals (ONO) in the CI-SD calculations was studied for 0, and SO, molecules and compared with the commonly used canonical virtual orbitals (CVOs). The systems studied represent a class of electron-rich molecules, in which the number of valence electron pairs exceeds substantially the number of formal chemical bonds. We found that the modified orbitals of the types studied appear to be less effective for these systems than in the similar calculations for the AH type molecules. Physical reasons for this difference were discussed. The evolution of spatial properties of virtual orbitals within the modification process was analyzed. 0 1996 John Wiley & Sons, Inc.
within the symmetry-restricted description, only approximately n / 2 of cp,'s are necessary for construction of the reference-state function, i.e., the function which does not describe dynaApical effects of electron correlation. The operator G generating this one-particle basis { q} can be orbital-independent, like, e.g., the bar-nuclear Hamiltonian. It can