Section Linear Response::KdotP
Method of calculating the contribution of the projection of the linear-response wavefunctions in the occupied subspace.
The Sternheimer equation is solved including the occupied subspace,
to get the full linear-response wavefunctions.
The Sternheimer equation is solved only in the unoccupied subspace,
and a sum-over-states perturbation-theory expression is used to
evaluate the contributions in the occupied subspace.