  Response calculations in density functional theory aim at computing the
change in ground-state density induced by an external perturbation. At finite
temperature these are usually performed by computing variations of orbitals,
which involve the iterative solution of potentially badly-conditioned linear
systems, the Sternheimer equations. Since many sets of variations of orbitals
yield the same variation of density matrix this involves a choice of gauge.
Taking a numerical analysis point of view we present the various gauge choices
proposed in the literature in a common framework and study their stability.
Beyond existing methods we propose a new approach, based on a Schur complement
using extra orbitals from the self-consistent-field calculations, to improve
the stability and efficiency of the iterative solution of Sternheimer
equations. We show the success of this strategy on nontrivial examples of
practical interest, such as Heusler transition metal alloy compounds, where
savings of around 40% in the number of required cost-determining Hamiltonian
applications have been achieved.
