All periodic calculations were performed using the Vienna ab initio package (VASP), employing the GGA-PBE functional and a plane-wave basis set, with the projector-augmented wave (PAW) method for the description of core electrons. All cluster calculations were performed using the ORCA package. We employed the PBE and B3LYP functionals, as well as the MP2 wavefunction-based method, all with the TZVP basis set. For the localized average ionization energy (ALIE), we employed the CCSD(T)/aug-cc-pV5Z level of theory.
Vacek et al. (Mon,) studied this question.