Magnetic exchange couplings from noncollinear spin density functional perturbation theory

Abstract

We propose a method for the evaluation of magnetic exchange couplings based on noncollinear spin density functional calculations. The method employs the second derivative of the total Kohn–Sham energy of a single reference state, in contrast to approximations based on Kohn–Sham total energy differences. The advantage of our approach is twofold: It provides a physically motivated picture of the transition from a low-spin to a high-spin state, and it utilizes a perturbation scheme for the evaluation of magnetic exchange couplings. The latter simplifies the way these parameters are predicted using first principles: It avoids the nontrivial search for different spin states that needs to be carried out in energy difference methods, and it opens the possibility of “black-boxifying” the extraction of exchange couplings from density functional theory calculations. We present proof of concept calculations of magnetic exchange couplings in the H–He–H model system and in an oxovanadium bimetallic complex where the results can be intuitively rationalized.