The effect of electronic correlation on molecular wavefunctions

Abstract

The minimal basis set Hartree-Fock and full-CI wavefunctions of some molecules (N 2, CO, BH 2, CH 4) are analyzed in terms of their valence-bond components. The internal valence correlation has dramatic effects on this distribution. The coefficients of the ionic forms decrease very rapidly with ionicity; the largest coefficients concern neutral determinants with maximum spreading of the p electrons which are multiplied by important factors (3 to 10) by the CI. The neutral components with two electrons is the same p AO are increased to a lesser extent. Correlation decreases the charge fluctuation in a dramatic way. On the other hand the atomic spin fluctuation increases under correlation. When the main atomic configuration is not the ground state one (as in BH 2 or CH 4) the correlation does not increase the components of the atomic ground states, and has little effect on hybridization. These various trends reflect a compromise between the necessity of bond building between electrons of opposite spins and the tendency of the atoms to increase their components in their low energy configurations.