Analyses of Chemical Bonds

Abstract

The analysis of a system composed of two or more interacting subsystems is often pursued, in ab initio studies, by taking as hamiltonian the same hamiltonian used for atomic systems but with the inclusion of more than one nucleus; the nuclei are kept at fixed positions as is done in atomic computations (Born-Oppenheimer approximation). Thus, the system contains as particles, the m nuclei and n electrons; we briefly designate it as the m,n-system. For example, in considering the interaction of NH₃ with HCl, a reaction path was obtained (53) by considering a number of different relative positions for the two subsystems, NH₃ and HCl, and by repeating standard SCF-LCAO-MO computations at each position; however, nowhere in the hamiltonian is there an explicit reference to the decomposition of the two subsystems. In recent years this method has been called “supermolecule approach”, a rather unnecessary and inappropriate name, in our opinion, since the term “molecule” has a very precise definition that conflicts with the implications in the term “supermolecule”.