Klaus Ruedenberg and Mark Gordon of Iowa State university are supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry division to develop a general method for resolving the bond energies in any molecule and between interacting molecules in terms of contributions that arise from the deformations of the individual atoms and from the interactions between different atoms. The latter are furthermore resolved in terms of contributions that are attributed to distinct physical interactions, viz. quasi-classical electrostatic interactions, interference interactions and charge transfer effects. The analysis is extracted from the accurate ab initio energy expression by transformation into a basis of quasi-atomic orbitals that are intrinsically deduced from the accurate ab initio wave function. The role of the various contributions and of their kinetic and potential energy components in the establishment of chemical bonding is investigated for a number of reactions. On this basis, conceptual characterizations of bonding are identified and a rigorous ab initio basis for similarities and distinctions between different bonding patterns is established.
Current quantum chemistry can generate quantitative information and predictions that are valuable and useful in many areas of chemical work and that the purely experimental approach is unable to supply. These capabilities are the result of advances in mathematical algorithms in conjunction with the steady increase in the speed and storage capacity of electronic computers. The increasing complexity of the computational techniques has made it, however, extremely difficult to perceive the physical reasons why bonds form and break between atoms, even in simple and fundamental chemical reactions. This lack of physical insight has created a large gap between the number crunching approach and the chemical intuition that has served experimental chemists well for two centuries, and the bridging between the two has proved to be nontrivial. The present work identifies the physical contents of these complex calculations and extracts an interpretation of chemical bonding that can fit into the intuitive frame work of chemical reasoning. Previously untapped chemically interesting information, which is imbedded in accurate quantum mechanical computations, is made accessible.
