Exchange-correlation magnetic fields in spin-density-functional theory

Abstract

In spin-density-functional theory for noncollinear magnetic materials, the\nKohn-Sham system features exchange-correlation (xc) scalar potentials and\nmagnetic fields. The significance of the xc magnetic fields is not very well\nexplored; in particular, they can give rise to local torques on the\nmagnetization, which are absent in standard local and semilocal approximations.\nWe obtain exact benchmark solutions for two electrons on four-site extended\nHubbard lattices over a wide range of interaction strengths, and compare exact\nxc potentials and magnetic fields with approximations obtained from\norbital-dependent xc functionals. The xc magnetic fields turn out to play an\nincreasingly important role as systems becomes more and more correlated and the\nelectrons begin to localize; the effects of the xc torques, however, remain\nrelatively minor. The approximate xc functionals perform overall quite well,\nbut tend to favor symmetry-broken solutions for strong interactions.\n