定域片断轨道基组的对称化

片断的UHF运算不能保证每个片断轨道具有确切的电子占据数,故Kost定域化是必需的.当片断产生于多键断裂时,在确保目标轨道单占据性的同时,Kost定域破坏了轨道基组的对称性.为此,在Kost定域化后,必须对单占据轨道作2×2对称化旋转后,再作有条件的RHF运算.以乙烯基片断CHCH(波二烯分子中的一个片断)为例,详述了对称化的方法、原理和计算程序.以C-H片断为例,细述三单键片断轨道基组对称化的特殊性.介绍C-HR参考键长选择的判据,探讨键长与选择Gaussian基组大小的关系.

Symmetrization of the Localized Fragment Molecular Orbital Basis

The UHFcomputation for a fragment resulted from the breaking of the single bonds can′t ensure that each of the opened-shell delocalized FMO s (fragment molecular orbitals) Ψ_i^ohas a correct electronic occupancy. It is necessary to localize Ψ_i^owith the Kost method. However, the Kost localization destroys the symmetry of the localized FMO basis set Ψ iin the case of fragments such as -CHCH- and CH that are the fragments of polynorbornyl diene and resulted from the breaking of two and three carbon-carbon single bonds respectively. In this work, the localized FMO basis Ψ_ifor ethylenic group was taken as an example, the procedures for symmetrizing Ψ_iare detailed. At a higher Gaussian basis level such as 3-21G, the method that determines the length r of the bond between carbon atom and its one referential hydrogen HR is introduced.