Jsymm: A Python package for symmetry analysis of exchange tensors in magnetic Hamiltonians
Symmetries of a crystal often restrict its physical properties. In particular, they determine possible forms of the tensors that describe interatomic exchange interaction, which governs a wide range of magnetic phenomena. Computationally demanding first-principles calculations of the exchange tensors can be greatly simplified by taking the symmetry constraints into account. Here, we present Jsymm, a Python package that derives the most general symmetry-compatible form of the exchange tensors directly from the crystallographic data. For any bond formed by magnetic ions, Jsymm produces the tensors of the Dzyaloshinskii-Moriya and anisotropic Heisenberg exchange interaction in symbolic form, as well as the tensors for all other bonds related to it by symmetry. This reduces the number of independent model parameters, dramatically lowering the computational cost of the ab initio calculations and preventing unphysical results arising from symmetry violations. The package accepts standard CIF files and provides a web interface in addition to an interactive text mode and a Python library. We demonstrate its utility on La$_2$CuO$_4$ and $α$-Fe$_2$O$_3$, reproducing known symmetry constraints and revealing additional relations between components of the exchange tensors of different bonds.