EDRIXS: an open source toolkit for simulating resonant inelastic x-ray scattering spectrum based on exact diagonalization

In this work, we present an open source toolkit called "EDRIXS" to simulate the resonant inelastic x-ray scattering (RIXS) spectrum based on exact diagonalization (ED) of a model Hamiltonian. It can deal with single atom, cluster models with the parameters being obtained by DFT+Wannier90 calculation, and Anderson impurity models obtained from a converged DFT+DMFT calculation. A very efficient parallel ED solver based on arpack library is implemented and RIXS spectrum is calculated by Krylov subspace technique. These key components are written by Fortran90 and a Python interface is designed to prepare the inputs and setup the calculations. We apply this toolkit to study two 5d materials. First, we perfectly reproduce the dimer excitations observed in the experimental RIXS spectrum of Ba₅AlIr₂O₁₁ based on a two site Ir-Ir model and it directly confirms the existence of dimer orbital in this material. Second, we simulate the measured RIXS spectrum in Ba₂YOsO₆ based on an Anderson impurity model from a DFT+DMFT calculation, and based on this simulation we found that the energy scale of spin-orbit coupling (SOC) and Hund's coupling is comparable and the effective SOC effects in this 5d³ compound is still significant so that it cannot be described by a pure spin-3/2 state.