Coulomb hole in N2, CO and O2deduced from X-ray scattering cross-sections

Accurate total (elastic and inelastic) X-ray scattering cross-sections of N2, CO and O2 were measured by the use of the energy dispersive method up to a momentum transfer of q = 12 au. The radial electron pair distribution function P(r 12) was extracted from the cross-sections. The Coulomb hole, defined as the difference between the exact P(r 12) and the corresponding function evaluated at the Hartree–Fock limit, has been derived from experimental data for the first time. Comparison of multi-reference configuration interaction (MRCI) and averaged quadratic coupled cluster (MR-AQCC) calculations indicates substantial shortcomings of MRCI due to the lack of size extensivity. The overall agreement with experiment is good but some differences between the theoretical and experimental results remain.