  Electrons in zero external magnetic field can be studied with density
functional theory (DFT) or with 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. Correcting this
error for open shells allows the approximate DFT xc functionals to become as
accurate as those in SDFT. In the limit of zero magnetic field, the Kohn-Sham
equations of SDFT emerge as the generalised KS equations of DFT.
