[gpaw-users] New finite-difference stencil for Poisson equation

[Jens Jørgen Mortensen]

Hi!

Now that GPAW can handle non-orthogonal unit cells, we need to change
the default finite-difference stencil for the Poisson equation.
Currently we use a Mehrstellen stencil for orthogonal cells and a
central FD stencil with n=2 neighbors in all directions for the
non-orthogonal case.

We need to use n=3 in the future so that we don't get a jump in the
energy when going from an orthogonal cell to a slightly non-orthogonal
cell.  The switch from Mehrstellen to n=3 will change total energies
slightly.  In my experience it is in the meV/atom range, but it will
depend on the system and the grid spacing used.

I think we should make the change as soon as possible so that it will be
in the upcoming version 0.7.

You could already now try to use poissonsolver=PoissonSolver(nn=3) in
your calculations and check how big the change is.

Jens Jørgen