Towards a constrained local moment model for first principles spin dynamics

We argue that, as originally formulated, the first principles spin dynamics of the finite-temperature and non-equilibrium properties of itinerant magnets recently proposed by Antropov el al. is not clearly defined within density functional theory. We show how constrained density functional theory can be used to provide a formal basis for describing the instantaneous non-collinear states that are being evolved according to the classical equation of motion of spin-dynamics. We propose a constrained local moment (CLM) model in which specific orientational configurations are maintained by local transverse constraining fields that are obtained self-consistently. A general algorithm for finding the constraining fields is used and the existence of a CLM state is demonstrated for specific model problems including a cell of 512 Fe atoms for which the orientations of the magnetic moments are random.