单占据片断轨道的组合式(Kost+Perkin)定域化

为了使开壳层片断轨道具有正确的集居数,正则轨道必须定域化.但是,在确保目标轨道Φ_sdan单占据性的同时,Kost定域化也破坏了Φ_s特定的对称性.实际计算表明,Perkin定域化可以弥补Kost程序的缺陷,将单占据的Φ_s转化成高度定域的、对称的片断轨道.在片断分子中,C-H_R键长r的选择和Kost定域化的方式对Perkin定域化的成败具有重大的影响.当Gaussian基组为STO-3G3-21G和4-31G时,r应为0.1nm;但在6-31G水平下,必须r=0.09nm.

Localization of a Combinatorial Procedure for Singly Occupied Fragment FMOs

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.