QE-CONVERSE: An open-source package for the Quantum ESPRESSO distribution to compute non-perturbatively orbital magnetization from first principles, including NMR chemical shifts and EPR parameters

Abstract: Orbital magnetization, a key property arising from the orbital motion of electrons, plays a crucial role in determining the magnetic behavior of molecules and solids. Despite its straightforward calculation in finite systems, the computation in periodic systems poses challenges due to the ill-defined position operator and surface current contributions. The modern theory of orbital magnetization, implemented in the Density Functional Theory (DFT) framework, offers an accurate solution via the "converse approach." Here, we introduce QE-CONVERSE, a refactored, modular implementation of this method, replacing outdated routines from Quantum ESPRESSO (version 3.2). QE-CONVERSE integrates modern computational libraries like scaLAPACK and ELPA, enhancing scalability, especially for large supercell calculations. This work focuses on providing the community with a reliable, accurate orbital magnetization package for properties such as Electron Paramagnetic Resonance (EPR) g-tensors and Nuclear Magnetic Resonance (NMR) chemical shifts, particularly where perturbative methods fail. We demonstrate QE-CONVERSE's effectiveness with benchmark cases, including the NMR shifts of ${}^{27}$Al in alumina and ${}^{17}$O and ${}^{29}$Si in $\alpha$-quartz, as well as the EPR g-tensor for $_{ }^{n}\Sigma(n\geq 2)$ radicals and nitrogen defects in silicon. Results show excellent agreement with theoretical and experimental data, with improved accuracy in EPR calculations over linear response methods. QE-CONVERSE is fully compatible with recent Quantum ESPRESSO versions, enabling new possibilities for studying complex materials