Escaping the symmetry dilemma through a pair-density interpretation of spin-density functional theory

In the standard interpretation of spin-density functional theory, a self-consistent Kohn-Sham calculation within the local spin density (LSD) or generalized gradient approximation (GGA) leads to a prediction of the total energy E, total electron density n(r)=${\mathit{n}}_{\mathrm{\uparrow }}$(r)+${\mathit{n}}_{\mathrm{\downarrow }}$(r), and spin magnetization density m(r)=${\mathit{n}}_{\mathrm{\uparrow }}$(r)-${\mathit{n}}_{\mathrm{\downarrow }}$(r). This interpretation encounters a serious ‘‘symmetry dilemma’’ for ${\mathrm{H}}_2$, ${\mathrm{Cr}}_2$, and many other molecules. Without changing LSD or GGA calculational methods and results, we escape this dilemma through an alternative interpretation in which the third physical prediction is not m(r) but the on-top electron pair density P(r,r), a quantity more directly related to the total energy in the absence of an external magnetic field. This alternative interpretation is also relevant to antiferromagnetic solids. We argue that the nonlocal exchange-correlation energy functional, which must be approximated, is most nearly local in the alternative spin-density functional theory presented here, less so in the standard theory, and far less so in total-density functional theory. Thus, in LSD or GGA, predictions of spin magnetization densities and moments are not so robust as predictions of total density and energy. The alternative theory helps to explain the surprising accuracy of LSD and GGA energies, and suggests that the correct solution of the Kohn-Sham equations in LSD or GGA is the fully self-consistent broken-symmetry single determinant of lowest total energy.