AN AB-INITIO MOLECULAR-FRAGMENT APPROACH TO LARGE POLYATOMIC MOLECULAR SYSTEMS

Abstract

An ab initio theory is presented that enables one to treat a polyatomic system by breaking it up into molecular fragments based on chemical intuition and carry out calculations on one of these fragments at a time. Unlike the Molecular-Fragment method of Christoffersen et al.,$^{1}$ the present method fragments even the SCF or MCSCP computations In the sense that in a given step one optimizes the orbitals for one molecular fragment only, with the rest of the polyatomic system serving as an environment. The greatest advantage of such an approach lies in the fact that since the orbitals belong to one or the other of the fragments, they are represented by basis functions which are centered on that particular fragment or its nearest neighbors. Thus, the number of integrals to be computed as well as the efforts needed for SCF-optimizations are drastically reduced. Expressions for the total energy, the orbital energies, and the formulae relating to the orbital optimizations are derived. The correlation problem under the present scheme is discussed. The method is illustrated by actual computation on the ethylene molecule.