SELF-CONSISTENT, NONORTHOGONAL GROUP-FUNCTION APPROXIMATION .3. APPROACHES FOR MODELING INTERMOLECULAR INTERACTIONS

A new program has been developed for the implementation of the self-consistent nonorthogonal group function (NOGF) approximation for the calculation of wavefunctions between interacting systems. The NOGF approach is based on the reformulation of a single-determinental, closed shell wavefunction into an antisymmetrized product of nonorthogonal group functions, each of which is a single determinant of doubly occupied orbitals. The group product form of the wavefunction combined with the relaxation of the orthogonality constraints and the structure of the orbital equations allows each group's orbitals to be expanded in a local basis set, and makes it possible to modify the orbital expansions or iterative process to simplify the computation. Three models for approximating the full single-determinental wavefunction are tested: (1) different quality basis sets for orbitals belonging to different groups, (2) variation of the types of intergroup interactions included in the wavefunction, and (3) the use of frozen orbitals which have been predetermined in a subsystem and are subsequently transferred to the system of interest. These approximations are applied to the calculation of protonation energies of formate, ammonia, imidazole, and guanidine with part of the first hydration shell being represented by two water molecules hydrogen bonded to each species. The results from different basis sets are compared. Then interaction potentials between the acid and ammonia are calculated for both neutral and charged forms, again with inclusion of part of the first hydration shell. The results show that these techniques can yield reliable wavefunctions of the same quality as obtained with the standard supermolecule approach. The effect of the basis set superposition error in the NOGF approach is briefly considered, and the reduction in computing effort resulting from the three models is discussed.