Which preconditioner to use in order to solve the Kohn-Sham
equations or the linear-response equations. The default is
pre_filter, except for curvilinear coordinates, where no
preconditioner is applied by default.
Options:
no:
Do not apply preconditioner.
pre_filter:
Filter preconditioner.
pre_jacobi:
Jacobi preconditioner. Only the local part of the pseudopotential is used.
Not very helpful.
pre_poisson:
Uses the full Laplacian as preconditioner. The inverse is calculated through
the solution of the Poisson equation. This is, of course, very slow.