A new method for modelling spectator chemical groups in ab initio calculations: effective group potentials

A new method for an increased numerical efficiency of ab initio calculation s is proposed. It is based on the assumption that in most cases chemical pr operties of functional groups in molecules are mainly controlled by a few e lectrons. This statement allows one to distinguish between two classes of n uclei and electrons: active and inactive ones. The effective group potentia l (EGP) method presupposes that the effect of inactive electrons in a funct ional chemical group can be described by a pseudopotential, in the same way that core electrons are replaced by effective core potentials in atoms. It is shown that EGPs are able to predict chemical and structural features of the active part of a molecule and at a fraction of the ordinary computatio nal cost. The preliminary results reported here concern the determination o f EGPs for ammonia, the methyl radical and the cyclopendadienyl ligand, whi ch represent different types of bonding.