Calculations of Multiplet Splittings in Open Shell Systems within the GW approximation

In open shell systems, the coupling of the orbital and spin angular momenta of electrons can result in multiple energy eigenstates of the electronically excited system leading to the characteristic multiplet structures observed in photoemission experiments. While DFT is not capable of determining the resulting multiplet splittings accurately, recent studies have shown that the GW approximation can describe the multiplet structure with reasonable accuracy¹. In this work, we investigate the multiplet splittings for open shell molecules such as NO₂, NF₂, O₂, ClO₂, and single atoms by applying the one-shot GW approximation. We compare predictions obtained with G and W computed with the same hybrid functional starting points containing varying amounts of Fock exchange with those where G is still computed with a hybrid functional starting point but W is computed within PBE. We show that it is possible to achieve excellent agreement with experimental results for molecular systems using both types of approaches, but the amount of exact exchange needed for quantitative accuracy depends on the molecule as well as the choice of the GW method employed.

¹J. Lischner, J. Deslippe, M. Jain, and S. G. Louie, Phys. Rev. Lett. 109, 036406 (2012).