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Accurate Valence Ionization Energies from Kohn–Sham Eigenvalues with the Help of Potential Adjustors

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journal contribution
posted on 07.08.2017, 00:00 by Adrian Thierbach, Christian Neiss, Lukas Gallandi, Noa Marom, Thomas Körzdörfer, Andreas Görling
An accurate yet computationally very efficient and formally well justified approach to calculate molecular ionization potentials is presented and tested. The first as well as higher ionization potentials are obtained as the negatives of the Kohn–Sham eigenvalues of the neutral molecule after adjusting the eigenvalues by a recently [Görling Phys. Rev. B 2015, 91, 245120] introduced potential adjustor for exchange-correlation potentials. Technically the method is very simple. Besides a Kohn–Sham calculation of the neutral molecule, only a second Kohn–Sham calculation of the cation is required. The eigenvalue spectrum of the neutral molecule is shifted such that the negative of the eigenvalue of the highest occupied molecular orbital equals the energy difference of the total electronic energies of the cation minus the neutral molecule. For the first ionization potential this simply amounts to a ΔSCF calculation. Then, the higher ionization potentials are obtained as the negatives of the correspondingly shifted Kohn–Sham eigenvalues. Importantly, this shift of the Kohn–Sham eigenvalue spectrum is not just ad hoc. In fact, it is formally necessary for the physically correct energetic adjustment of the eigenvalue spectrum as it results from ensemble density-functional theory. An analogous approach for electron affinities is equally well obtained and justified. To illustrate the practical benefits of the approach, we calculate the valence ionization energies of test sets of small- and medium-sized molecules and photoelectron spectra of medium-sized electron acceptor molecules using a typical semilocal (PBE) and two typical global hybrid functionals (B3LYP and PBE0). The potential adjusted B3LYP and PBE0 eigenvalues yield valence ionization potentials that are in very good agreement with experimental values, reaching an accuracy that is as good as the best G0W0 methods, however, at much lower computational costs. The potential adjusted PBE eigenvalues result in somewhat less accurate ionization energies, which, however, are almost as accurate as those obtained from the most commonly used G0W0 variants.