定域片断轨道基组的对称化

Symmetrization of the Localized Fragment Molecular Orbital Basis

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 CH CH(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 .

The UHF computation for a fragment resulted from the breaking of the single bonds can′t ensure that each of the opened-shell delocalized FMOs (fragment molecular orbitals) Ψ o i has a correct electronic occupancy. It is necessary to localize {Ψ o i} with the Kost method. However, the Kost localization destroys the symmetry of the localized FMO basis set {Ψ i}in 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 i} for ethylenic group was taken as an example, the procedures for symmetrizing {Ψ i} are 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 H R is introduced.