Electron correlation, reference states and empirical potentials

Abstract

Reference states as used in the physical sciences fall into three main categories: simplified interactions, special limits or cases, and special symmetries or configurations. Regardless of the category, the general behaviour of a system is described as deviations from the specific behaviour of some reference state. After briefly reviewing examples from each of the categories, we more closely examine the role of reference states in constructing empirical atomistic potential energy surfaces. Although not universally used for parameterizing empirical potentials, we argue that the approach deserves more consideration based on the success of the embedded atom method (EAM), and its variants. We view a substantial part of the success of EAM as due to its use of a reference state. We further argue that one role of the reference state is to introduce correlation energy in a fundamental way. We take advantage of these characteristics in deriving a generalization of EAM that permits a description of charge transfer (CT-EAM). The generalization is based on a rigorous analysis of a valence bond model. The CT-EAM model introduces the charge of the reference state in a role that is analogous to the reference-state energy in the original EAM. The reference-state charge enables the model to switch between linear and quadratic dependence of the energy on charge, during a chemical reaction, as demanded by fundamental results from density functional theory.

Acknowledgments

The authors acknowledge support from National Science Foundation grant No. CHE-0304710 at the University of New Mexico. In addition, the work of SMV was performed in part at Los Alamos National Laboratory under the auspices of the US Department of Energy, under contract No. W-7405-ENG-36. SRA is grateful the support of the National Science Foundation under grant No. DMR-9520371 during the initial stages of this work. The authors would like to acknowledge numerous insightful communications with Michael I. Baskes, Paul W. Ayers, Robert G. Parr, Thomas P. Swiler, and Todd J. Martinez.