On the group function model for molecular electronic structure

A group function model for the electronic structure of molecules is developed in which the strong orthogonality condition is not necessarily invoked. The group functions are required to satisfy orthogonality constraints which are less restrictive than the strong orthogonality condition. The particular condition which two group functions obey reflects the extent to which the electrons they describe are localized in well-separated regions of space and the degree of interaction between them. This will be called the interacting group function model. General expressions for the expectation values of one and two-electron spin-independent operators are derived using the spin-coupled formalism. A graphical technique is found to be useful when the molecule is divided into several subsystems.