An Extended CCSD Model Including Triple Excitations for Core-Ionization Energies

A model for calculation of core-ionization energies is developed within the equations-of-motion coupled cluster framework with application of the CVS approximation. The model deviates from EOM-CCSD by inclusion of triple projection manifold and triple excitation operators, as well as triple excitation amplitudes. However, the triple projection manifold is restricted to only include states involving a specific core excitation, namely an excitation to a super-diffuse orbital. As the super-diffuse orbital does not interact with the molecule orbitals, the model produces core-ionization energies. The truncation level of the cluster operator remains as in EOM-CCSD, but a trivial zero triple cluster operator is added. There are still some minor errors present in the pilot code, but it is expected that the model produces results at an accuracy level close to EOM-CCSDT. The computational cost, however, is reduced to n^7 compared to EOM-CCSDT, which scales as n^8, and could be further reduced to n^6, n denoting the number of basis functions. Since the proposed model is the theoretical equivalent to the experimental procedure XPS, it is named XPS-CCSD. Theoretical spectra may be constructed by core-ionization energies obtained by the XPS-CCSD model, where these are helpful in the interpretation of their experimental counterpart