MODIFICATIONS OF VIRTUAL ORBITALS IN THE LIMITED CI CALCULATIONS FOR ELECTRON-RICH MOLECULES

The efficiency of modified virtual orbitals (MVO) of ionic type and of approximate orthogonalized natural orbitals (ONO) in the CI-SD calcul ations was studied for O-3 and SO2 molecules and compared with the com monly used canonical virtual orbitals (CVOs). The systems studied repr esent a class of electron-rich molecules, in which the number of valen ce 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 calculatio ns for the AH(n) type molecules. Physical reasons for this difference were discussed. The evolution of spatial properties of virtual orbital s within the modification process was analyzed. (C) 1996 John Wiley & Sons, Inc.