Insights into theoretical quantum chemistry from electron momentum spectroscopy

Most basis sets used in quantum chemistry are designed to get the correct charge and momentum density in the region important for covalent bonding. The (e,2e) cross section measured by electron momentum spectroscopy (EMS) emphasizes the low-momentum, large r, region of the wave function. Improving the description of this part of the wave function for water has resulted in good agreement with (e,2e) data. Because the hydrogen bond is sensitive to the long-range tail of the wave function, this has simultaneously led to an improved description of the hydrogen bond in the water dimer. The satellite region of the binding energy spectrum gives information about the excited states of the cation that is not available at present from any other form of spectroscopy. Calculations seeking agreement with the binding-energy spectra and the momentum distribution associated with satellite peaks have led to the most complete catalog of the cation excited states for ethylene. Here we report the assignment of the excited states based on the dominant part of the wave function rather than focusing on the small coefficients that describe the intensity borrowing from the primary holes. We also examine the adequacy of the assumption that every Dyson orbital is similar to one of the Hartree-Fock orbitals.