Implementation of spin-orbit coupling in the Real-space MultiGrid (RMG) code

Spin-orbit coupling (SOC) plays an important role in materials containing heavy atoms, such as the emerging topological materials and f-electron metals. We implemented spin-orbit coupling in the open-source DFT electronic structure package RMG, www.rmgdft.org, which scales well from workstations to the latest CPU-GPU supercomputers. Fully relativistic pseudopotentials are used to describe the core electrons either in norm-conserving or ultrasoft form [1]. The coupling between the spin-up and spin-down electrons is mediated by the non-local pseudopotentials of the different spin components. Non-collinear magnetism, present in many multiferroic materials, is handled through a similar formalism. The calculated band structure of bulk Pt agrees very well with the result from the plane-wave based Quantum Espresso. We have also studied the band gap dependence on the thickness of thin-films of the topological material MnBi₂Te₄, in calculations that included up to 5,200 electrons. Our results are in good agreement with those obtained by VASP. This implementation enables us to explore effects of local- as well as extended-heterogeneities in thin-films of topological quantum materials and compare with experimental measurements.
[1] A. D. Corso and A. M. Conte, PRB 71, 115106 (2005)