Density functionals with spin-density accuracy for open shells

Electrons in zero external magnetic field can be studied with the Kohn-Sham (KS) scheme of either density functional theory (DFT) or spin-DFT (SDFT). The latter is normally used for open-shell systems because its approximations appear to model better the exchange and correlation (xc) functional, but also because, so far the application of DFT implied a closed-shell-like approximation. In the first part of this Communication, we show that correcting this error for open shells allows the approximate DFT xc functionals to become as accurate as those in SDFT. In the second part, we consider the behavior of SDFT for zero magnetic field. We show that the KS equations of SDFT emerge as the generalized KS equations of DFT in this limit, thus establishing a so far unknown link between the two theories. (C) 2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/).

Automated summary

This paper investigates the behavior of electrons in zero external magnetic field using the Kohn-Sham scheme of density functional theory (DFT) or spin-density functional theory (SDFT). It is found that correcting the closed-shell approximation for open-shell systems allows the approximate DFT xc functionals to be as accurate as those in SDFT. Additionally, it is discovered that the KS equations of SDFT emerge as the generalized KS equations of DFT in the zero magnetic field limit, establishing a previously unknown link between the two theories.