Exchange and Correlation in Density Functional Theory and Quantum Chemistry

The nature of exchange, dynamic correlation (DC) and left-right correlation (LRC) is considered in density functional theory and wavefunction-based quantum chemistry. The presence of LRC in approximate exchange density functionals is highlighted and the separation of LRC and DC is considered. For H2, the Heitler-London approach is shown to include the essential elements of exchange and LRC. The arguments are illustrated by a comparison of Gaussian orbital s-optimised Heitler-London and OPTX potential energy curves. They agree well near equilibrium, but differ at large distances due to the inability of the OPTX form to describe the dissociation process. LRC and DC values determined using the two approaches are compared. The influence of higher angular momentum functions in the Heitler-London approach is then investigated (commonly called self-consistent valence bond); the agreement with OPTX degrades, leading to a larger value of LRC and a smaller value of DC at H-2 equilibrium. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 111: 563-569, 2011