Effective electronic Hamiltonian for quantum subsystem in hybrid QM/MM methods as derived from APSLG description of electronic structure of classical part of molecular system

The general formulae representing separation of electronic variables of quantum (reactive) subsystem from those describing electrons in the classical (chemically inert) part of molecular system are specified for the case when the electronic structure of the latter is described by a semi-empirical method based on the trial wave function having the form of antisymmetrized product of strictly localized geminals (APSLG) which leads to a local description of molecular electronic structure in terms of bond functions and lone pair functions. This allowed us to give an explicit form of the effective electronic Hamiltonian for the quantum subsystem and also by this to sequentially derive the explicit form of the QM/MM junction between the quantum and classical subsystems. The latter turned out to be a sum of the contributions from different chemical bonds and lone pairs residing in the classical part of the system. Numerical estimates for the effect of the renormalization of the Coulomb interaction of pi-electrons due to presence of a-bonds are performed according to the derived formulae. (C) 2000 Elsevier Science B.V. All rights reserved.