A new partitioning scheme for molecular interacting systems within a multiconfigurational or monoconfigurational Hartree-Fock formalism

A new method, based on the spatial decomposition of the reduced-density and pair-density matrices and the indistinguishable integrals formalism, is in troduced to partition the molecular and stabilization energies into meaning ful fragments. These are defined as entirely flexible variable-size entitie s, for example, atoms, group of atoms, ions, and interacting monomers. This new partitioning scheme is especially appropriated to study systems in whi ch a directly bonded group-transfer process occurs. In these cases, the sta bilization energies are partitioned into an intrafragment component, associ ated with the difference of intrinsic affinity to the transferred group bet ween the involved fragments, and an interfragment component, associated wit h the difference of the magnitude of the interaction between the fragments in the initial and final binding complexes, This method was applied to the study of the arginine-carboxylate interactions, allowing us to have insight into what really happens in this system. Two (zwitterionic and neutral) bi nding complexes can be considered. The main effects accountable for the pre ferential stabilizations of the binding complexes are determined to be basi s-set independent. The zwitterionic complex is favored by the interfragment component, while the neutral complex is favored by the larger intrinsic pr oton affinity of the acetate relatively to the methylguanidium. (C) 1999 Jo hn Wiley & Sons, Inc.