Electron correlation effects in N2 and CO studied by X-ray scattering and CISD calculations

Total X-ray scattering intensities σ_ee(q) for N₂ and CO have been measured as a function of momentum transfer using the energy dispersive method. Novel procedures to extract accurate σ_ee(q), which eliminate effects of polarization, inelastic scattering, anomalous dispersion, and molecular vibration, have been proposed. A simplified theoretical treatment based on configuration interaction singles and doubles (CISD) calculations has been suggested. This procedure makes it possible to apply combined theoretical and experimental X-ray scattering studies to larger molecules. The inclusion of f and g functions is crucial, and the σ_ee(q) calculated with the cc-pVQZ[5s4p3d2f1g] basis set almost reproduces the data based on more elaborate MR-CISD calculations within the experimentally most relevant region of up to q ? 3 au. In contrast to experimental electron scattering data, the X-ray scattering intensities agree well with the computed results.