Group functions, Lowdin partition, and hybrid QC/MM methods for large molecular systems

The problem of developing an exact form of the junction between the quantum and classical parts in a hybrid QC/MM approach is considered. We start fro m the full Hamiltonian for the whole system and assume a specific form of t he electron wavefunction, which allows us to separate the electron variable s relevant to the reactive (quantum) part of the system from those related to the inert (classical) part. Applying the Lowdin partition to the full Ha miltonian for the molecular system results in general formulae for the pote ntial energy surfaces of a molecular system composed of different parts pre sided some of these parts are treated quantum mechanically whereas others a re treated with use of molecular mechanics. These principles of separating electron variables have been applied to construct an efficient method for a nalysis of electronic structure and d-electron excitation spectra of transi tion metal complexes. This method has been also combined with the MM approx imation in order to get a description for potential energy surfaces of the complexes and to develop a consistent approach to the known problem of exte nding molecular mechanics to transition metals.