Magnetization dynamics with time dependent SDFT: Significance of exchange-correlation torques

In spin-density functional theory (SDFT) for noncollinear magnetic materials, the Kohn-Sham system features exchange-correlation (xc) scalar potentials and xc magnetic fields, which account for many-body interaction effects. The significance of the xc magnetic fields is not very well explored; in particular, they can give rise to local torques on the magnetization which are absent in standard local and semilocal approximations. Exact solutions for a five-site extended Hubbard model at half filling and in the presence of spin-orbit coupling are compared with SDFT results obtained using orbital-dependent exchange-only approximations. The magnetization dynamics following short-pulse excitations is found to be reasonably well described in the exchange-only approximation for weak to moderate interactions. For stronger interactions and near transitions between magnetically ordered and frustrated phases, exchange and correlation torques tend to counteract.