A scaled CIS(D) based method for the calculation of valence and core electron ionization energies

The calculation of electron ionization energies is a key component for the simulation of photoelectron spectroscopy. CIS(D) is a perturbative doubles correction for the single excitation configuration interaction (CIS) method which provides a new approach for computing excitation energies. It is shown that by introducing a virtual orbital subspace that consists of a single "ghost" orbital, valence electron ionization energies can be computed using a scaled CIS(D) approach with an accuracy comparable with considerably more computationally intensive methods, such as ionization-potential equation of motion coupled cluster theory, and simulated spectra show a significant improvement relative to spectra based upon Koopmans' theorem. When the model is applied to the ionization energies for core orbitals, there is an increase in the error, particularly for the heavier nuclei considered (silicon to chlorine), although the relative energy of the ionization energies are predicted accurately. In addition to its inherent computational efficiency relative to other wavefunction based approaches, a significant advantage of this approach is that the ionization energies for all electrons can be obtained in a single calculation, in contrast to Δself-consistent field based methods.