Coulomb hole in N₂, CO and O₂ deduced from X-ray scattering cross-sections

Accurate total (elastic and inelastic) X-ray scattering cross-sections σ_ee(q) 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(r₁₂) was extracted from the cross-sections. The Coulomb hole, defined as the difference between the exact P(r₁₂) 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.