Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Open-shell systems are normally described using spin density
functional theory (SDFT) rather than regular DFT, due to spinpolarised approximations appearing to yield superior results
for exchange-correlation (xc) energies. To address the seemingly poorer (xc) energies obtained via DFT approximations
(DFAs), we show that correcting for a qualitative error in the
DFT description for open-shell systems, one obtains results
with SDFT accuracy. Furthermore, in the absence of external magnetic fields, both DFT and SDFT should reduce to
the same limit. We provide the link between these two theories, demonstrating how the regular KS equations of SDFT
reduce to a new (generalised) set of KS equations for DFT in
this limit. We also extend these ideas to ensembles of varying
electron number, obtaining a finite derivative discontinuity for
commonly used (semi-)local DFAs.