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The Shirley reduced basis: a reduced order model for plane-wave DFT

The Shirley reduced basis (SRB) represents the periodic parts of Bloch functions as linear combi- nations of eigenvectors taken from a coarse sample of the Brillouin zone, orthogonalized and reduced through proper orthogonal decomposition. We describe a novel transformation of the self-consistent density functional theory eigenproblem from a plane-wave basis with ultra-soft pseudopotentials to the SRB that is independent of the k-point. In particular, the number of operations over the space of plane-waves is independent of the number of k-points. The parameter space of the transformation is explored and suitable defaults are proposed. The SRB is shown to converge to the plane-wave solution. For reduced dimensional systems, reductions in computational cost, compared to the plane-wave calculations, exceed 5x. Performance on bulk systems improves by 1.67x in molecular dynamics-like contexts. This robust technique is well-suited to efficient study of systems with strin- gent requirements on numerical accuracy related to subtle details in the electronic band structure, such as topological insulators, Dirac semi-metals, metal surfaces and nanostructures, and charge transfer at interfaces with any of these systems. The techniques used to achieve a k-independent transformation could be applied to other computationally expensive matrix elements, such as those found in density functional perturbation theory and many-body perturbation theory.