Abstract: In order to ensure that each of all opened shell-fragment MOs (FMO Φ_i), obtained from the UHFcomputation for a molecular fragment, has a correct electronic occupancy, the FMOs Φ_i have to be localized. However, the specified symmetrization of the singly occupied FMOs (Φ_s) is destroyed while Φ_s becomes singly occupied after the Kost localization. The Perkin localization, a computational procedure after the Kost localization, transforms Φ_s into Φ_s which possesses now the symmetrization which they should have. The way of the Kost localization and setting of the length of the bond C-H_Rbetween the carbon atom and its referential hydrogen atom H_Rhave great effects on the symmetric characters of the localized FMOs obtained from the Perkin localzation. In the case of the Gaussian basis sets such as STO-3G, 3-21Gand 4-31G, the bond length r should be set to 0.1 nm, and when₆-31Gis used, it should be 0.09 nm.

Key words: The Kost localization, The Perkin localization, Fragment molecular orbital, Fragmental molecule