A study of the valence orbitals of H2S by electron momentum spectroscopy: Quantitative comparisons using Hartree-Fock limit and correlated wavefunctions

Reference Energies for Valence Ionizations and Satellite Transitions

Upon ionization of an atom or a molecule, another electron (or more) can be simultaneously excited. These concurrently generated states are called "satellites" (or shakeup transitions) as they appear in ionization spectra as higher-energy peaks with weaker intensity and larger width than the main peaks associated with single-particle ionizations. Satellites, which correspond to electronically excited states of the cationic species, are notoriously challenging to model using conventional single-reference methods due to their high excitation degree compared to the neutral reference state. This work reports 42 satellite transition energies and 58 valence ionization potentials (IPs) of full configuration interaction quality computed in small molecular systems. Following the protocol developed for the quest database [Véril, M.; Scemama, A.; Caffarel, M.; Lipparini, F.; Boggio-Pasqua, M.; Jacquemin, D.; and Loos, P.-F. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2021, 11, e1517], these reference energies are computed using the configuration interaction using a perturbative selection made iteratively (CIPSI) method. In addition, the accuracy of the well-known coupled-cluster (CC) hierarchy (CC2, CCSD, CC3, CCSDT, CC4, and CCSDTQ) is gauged against these new accurate references. The performances of various approximations based on many-body Green's functions (GW, GF2, and T-matrix) for IPs are also analyzed. Their limitations in correctly modeling satellite transitions are discussed.

Orbitals observed by Electron Momentum Spectroscopy

Numerical representations of Dyson orbitals can be extracted from the experimental data of electron momentum spectroscopy (EMS). These orbitals are solutions of the quasiparticle equation for the target electronic system. They closely obey the weak-coupling approximation. This defines a normalized Dyson orbital in terms of the sum of EMS cross sections for an orbital manifold of ion states whose momentum profiles have the same shape. Normalized Dyson orbitals are closely related to the orbitals of density functional theory. They are realistic in the sense that they give an independent-particle approximation to the target structure that reproduces experimental data, including molecular data obtained from experiments independent of EMS.