Constraining spin directions in density functional theory calculations by imposing a local magnetic field

By constraining the directions of magnetic moments in first-principles calculations, one can investigate the physical properties of desirable magnetic structures and extract parameters of magnetic interactions. However, the current methods for constraining the directions of magnetism in first-principles calculations may lead to quenched magnetic moments mathematically and practically. In this paper, we propose a method to constrain the spin directions in both collinear and noncollinear cases by imposing a local parallel magnetic field. The effectiveness of this method is confirmed by performing calculations on ${\mathrm{GdMn}}_{2}{\mathrm{O}}_{5}$ and MnSi by using a modified vasp package. Through these examples, we find that by adding an external parallel local magnetic field, we are able to constrain the spin directions more efficiently than existing methods.