Abstract
This paper summarises the theory and functionality behind Questaal, an open-source suite of codes for calculating the electronic structure and related properties of materials from first principles. The formalism of the linearised muffin-tin orbital (LMTO) method is revisited in detail and developed further by the introduction of short-ranged tight-binding basis functions for full-potential calculations. The LMTO method is presented in both Green’s function and wave function formulations for bulk and layered systems. The suite’s full-potential LMTO code uses a sophisticated basis and augmentation method that allows an efficient and precise solution to the band problem at different levels of theory, most importantly density functional theory, LDA+U, quasi-particle self-consistent GW and combinations of these with dynamical mean field theory. This paper details the technical and theoretical bases of these methods, their implementation in Questaal, and provides an overview of the code’s design and capabilities. Program summaryProgram Title: QuestaalProgram Files doi:http://dx.doi.org/10.17632/35jxxtzpdn.1Code Ocean Capsule:https://doi.org/10.24433/CO.3778701.v1Licensing provisions: GNU General Public License, version 3Programming language: Fortran, C, Python, ShellNature of problem: Highly accurate ab initio calculation of the electronic structure of periodic solids and of the resulting physical, spectroscopic and magnetic properties for diverse material classes with different strengths and kinds of electronic correlation.Solution method: The many electron problem is considered at different levels of theory: density functional theory, many body perturbation theory in the GW approximation with different degrees of self consistency (notably quasiparticle self-consistent GW) and dynamical mean field theory. The solution to the single-particle band problem is achieved in the framework of an extension to the linear muffin-tin orbital (LMTO) technique including a highly precise and efficient full-potential implementation. An advanced fully-relativistic, non-collinear implementation based on the atomic sphere approximation is used for calculating transport and magnetic properties.
Highlights
✩ This paper and its associated computer program are available via the Computer Physics Communication homepage on ScienceDirect
We denote localised basis sets as ‘‘KKR’’, for the Korringa-Kohn-Rostocker method [3] as it plays a central role in this work; but there are other kinds, for example the Gaussian orbitals widely favoured among quantum chemists. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The all-electron basis sets ‘‘APW’’ and ‘‘KKR’’ [3] are both instances of augmented-wave methods: both generate arbitrarily accurate solutions for a muffin-tin potential. They differ in their choice of envelope functions, but they are similar in that they join onto solutions of partial waves in augmentation spheres
Summary
✩ This paper and its associated computer program are available via the Computer Physics Communication homepage on ScienceDirect (http://www. sciencedirect.com/science/journal/00104655). Blöchl’s immensely popular Projector Augmented-Wave method [4] makes a construction intermediate between pseudopotentials and APWs. Questaal uses atom-centred envelope functions instead of plane waves (Section 3), and an augmentation scheme that resembles the PAW method but can be converged to an exact solution for the reference potential, as Slater’s original method did. The all-electron basis sets ‘‘APW’’ (augmented plane wave) and ‘‘KKR’’ [3] are both instances of augmented-wave methods: both generate arbitrarily accurate solutions for a muffin-tin potential They differ in their choice of envelope functions (plane waves or Hankel functions), but they are similar in that they join onto solutions of partial waves in augmentation spheres. Questaal can add APW’s to the basis to converge it to the LAPW standard (Section 3.10)
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