The Theoretical and Computational Chemistry program is supporting Prof. K. Ruedenberg at Iowa State University. The research to be carried out over the next two years will focus on the development of a new quantum chemical theory that will result in corresponding new computational chemistry codes. The objective is the quantitative determination of atoms in molecules within the framework of rigorous ab-initio theory, and the quantitative expression of binding energies in terms of interactions between such atoms. The deformed atoms are determined through their optimal wave functions in terms of orbitals restricted to the full-valence-space MCSCF molecular orbital space. Oriented quasiatomic orbitals, maximally involved in specific bonds and with unambiguous electron populations, are determined through a density matrix analysis. The binding energy appears as the sum of contributions due to intra-atomic promotion and inter-atomic non-bonded repulsion, electron sharing and charge transfer. The energy gain due to charge transfer is identified by determining the optimal molecular MCSCF wave function without charge transfer. The covalent energy gain is derived from the configurations representing electron migrations between atoms. The analysis applies to non-equilibrium as well as to equilibrium geometries and will be extended beyond the full valence space. Although for two centuries chemical experimentation has been effectively guided by the concept of molecules being built from atoms, major unsolved problems have persisted to this day regarding the theoretical foundation of this well established empirical model and it has not yet been quantified within the context of rigorous quantum mechanical calculations. This is because the atoms-in-molecules concept is, in fact, not a fundamental one in basic theory. Recently, the Ruedenberg research group has developed a new analysis that is able to identify atomic sub-units in rigorous molecular calculations. With the help of this theoretical approach, atoms, as well as the interactions between them, will be quantitatively determined in molecules. The method to be developed is expected to make the results of theoretical computations more useful for practical chemical work by relating them more directly to the intuitive reasoning of experimentalists.
