Michael J. G Peach, David J. Tozer, Nicholas C. Handy, Int. J. Quantum Chem., 111, 563–569, 2011.
In my final collaboration with the renowned British theoretical chemist, Prof. Nicholas Handy, we investigated the nature of exchange and correlation, and how the definitions in DFT and QC are related. We developed definitions for left–right and dynamic correlation. No existing functionals are able to satisfy the constraints we determined through an examination of Heitler–London valence bond wavefunctions (which can be related to multi-configuration SCF wavefunctions).
Further information, including details of subsequent work in this area, can be found on the research page. For the abstract, and access to the full text, see below.
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 H2 equilibrium.