Questaal: A package of electronic structure methods based on the linear muffin-tin orbital technique

Questaal is a set of codes developed for calculating the electronic structure of materials from first principles. Developed by Mark van Schilfgaarde and his collaborators, Questaal descends from the work of Ole K. Andersen and his group in Stuttgart during the 80's on the linear muffin-tin orbital (LMTO) method. This is an atom-centred basis set that is by construction particularly efficient for solving the band structure problem. As Questaal has evolved, a variety of codes and techniques aimed at addressing both different material problems and implementing different levels of theory have been developed. These include methods based on the atomic sphere approximation, where a simplified description of the crystal potential can be exploited to facilitate calculations that are scarcely possible otherwise: recent calculations of the supercurrent in Josephson junction devices formulated in terms of non-equilibrium Green's functions calculated ab initio (using density functional theory) demonstrate that this method continues to be extremely useful. At the other end of the spectrum, highly accurate calculations for more basic material studies can be conducted using Questaal's implementation of quasiparticle self-consistent GW (QSGW). QSGW is a way to achieve a measure of self-consistency in Hedin's perturbative GW method, thereby solving problems associated with the starting point dependence of GW and increasing its reliability significantly. QSGW provides a fully ab initio description of a broad range of materials that is generally much better than density functional theory or its common extensions (eg LDA+U). Questaal also includes the ability to extend QSGW to problems with stronger correlation by adding ladder diagrams (via the Bethe-Salpeter equation), or by coupling QSGW with DMFT, these representing the current state-of-the-art. This talk will describe the background, capabilities, some methodological aspects, and future directions of the Questaal project.