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

Abstract

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 from the full Hamiltonian for the whole system and assume a specific form of the electron wavefunction, which allows us to separate the electron variables relevant to the reactive (quantum) part of the system from those related to the inert (classical) part. Applying the Löwdin partition to the full Hamiltonian for the molecular system results in general formulae for the potential energy surfaces of a molecular system composed of different parts provided some of these parts are treated quantum mechanically whereas others are treated with use of molecular mechanics. These principles of separating electron variables have been applied to construct an efficient method for analysis of electronic structure and d-electron excitation spectra of transition metal complexes. This method has been also combined with the MM approximation in order to get a description for potential energy surfaces of the complexes and to develop a consistent approach to the known problem of extending molecular mechanics to transition metals.