  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.
