ANALYSIS OF THE EFFECT OF ELECTRON CORRELATION ON CHARGE-DENSITY DISTRIBUTIONS

An analysis of charge density distributions is presented for some simple systems with several bond types. Correlated densities from Moller-Piesset perturbation theory to second-order (MP2), configuration interaction with single and double excitations (CISD) and quadratic CI (QCI) were obtained from the Z density, an effective density matrix which represents the response of the correlated system to any one-electron perturbation. These densities are compared with densities derived by integrating the CI wave function and with SCF densities. The difference between the Z density and the unrelaxed density is usually small, but it is significant in some instances. The change in density is exaggerated by MP2 compared to CISD or QCI. The unusual cases of CO (in which different correlation methods give substantially different dipoles) and malonaldehyde enol (in which the hydrogen bond geometry changes significantly on inclusion of correlation) are examined in detail.