Generalized Hybrid Orbital (GHO) Method for Combining Ab Initio Hartree−Fock Wave Functions with Molecular Mechanics

Abstract

The generalized hybrid orbital (GHO) method provides a way to combine quantum mechanical (QM) and molecular mechanical (MM) calculations on a single molecular system or supramolecular assembly by providing an electrostatically stable connection between the QM portion and the MM portion. The GHO method has previously been developed for semiempirical molecular orbital calculations, on the basis of neglect of diatomic differential overlap (GHO−NDDO); in the present work, it is extended to the ab initio Hartree−Fock (HF) level (GHO−AIHF). First, the theoretical foundation for the GHO−AIHF extension is discussed, and four different approaches are proposed to overcome the nonorthogonality between active molecular orbitals (MOs) and auxiliary MOs. In the first scheme, the auxiliary hybrid basis functions are projected out of the active QM basis. The second scheme neglects the diatomic differential overlap between the auxiliary basis and the active QM basis. In the third scheme, hybrid orbitals are constructed fr...