Compressed Modes for Topological Insulators using Eigenspace Projection

In general, Wannier functions are localized real space functions that are defined through a unitary transformation of eigenfunctions. Here we focus on a particular variant of Wannier functions, known as compressed modes (CMs), which are traditionally obtained from the minimization of the total energy plus an L1 regularization term. We demonstrate the difficulties that arise when calculating CMs for tight-binding models of topological insulators and suggest a new approach for calculating CMs by defining an objective functional that uses eigenspace projection.