Average and directional Compton profiles for the N2, O2, and CH2O molecules. I. Effect of electron correlation

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Abstract
The average Compton profile, and the Compton profiles along directions parallel to the bonds and perpendicular to the plane of each of the N2, O2, and CH2O molecules, were calculated at two levels of approximation. The uncorrelated charge densities were obtained from wavefunctions calculated at the self‐consistent‐field (SCF) level and correlation was introduced by means of configuration interaction (CI). The CI wavefunctions were constructed by accepting all single excitations plus a selected set of double excitations from an initial reference list of configuration accouting for 40%–50% of the correlation energy for each of the molecules. Double‐zeta and double‐zeta‐plus‐polarization quality bases constructed from Gaussian‐lobe functions were used to generate these wavefunctions. The effect of electron–electron correlation on the charge density and its anisotropies was studied by a comparison of the various profiles at the two levels. It is found that this interaction, similar to its effect in atomic systems, tends to broaden the SCF profiles and this tendency is attributed to the increased antibonding character in the momentum charge distribution introduced via CI. The effect is most pronounced on the profiles along directions parallel to bonds, least perpendicular to the plane of the molecule and intermediate for the average, and is found to be most dominant in the momentum regions affected by bonding. It is also found that the inclusion of electron‐correlation uniformly brings the theoretical results into better agreement with the experimental measurements when the latter are available. These results are analyzed in detail and an attempt is made to explain them.