On combining quantum chemistry and molecular mechanics. The potential energy surface of complexes of transition metals exemplified by spin transitionin cis-[Fe(bipy)(2)(CNS)(2)]

A new approach to calculating potential energy surfaces (PEP) of various sp in states of transition-metal complexes is suggested. The approach is based on description of the electronic structure of a complex within the method of effective crystalline field (ECF) ire which the wave function is written as an anti-symmetrized product of the wave function of d-electrons calcula ted in the approximation of configurational interaction and the function of ligand electrons determined in the self-consistent field approximation. Wi thin the ECF method an expression for the total energy of a molecule writte n in the form of a sum of the effective energy of d-electrons and ligands a pproximated by the molecular mechanics method (MM) is derived. The suggeste d approach is used to construct PES sections of various spin states of cis- Fe(bipy)(2)(NCS)(2) molecules along the straight line that connects the poi nts pertaining to spin isomers in the configuration space. The MM parameter s evaluated for iron atoms enable the total-energy terms to be reproduced i n such a manner that they tit the available experimental characteristics of spin transitions.