NpT - grid GPW

[Marc Van Houteghem]

Dear all,

I have a question concerning the GPW method, more specific about how the
regular grid used in the computation of the electronic density, is
generated.  In the following reference:
ChemPhysChem, 2005, 6, 1894
it is found that when using an (almost) constant density for the grid
points, one should take the cut-off high enough (i.e. a 1200 Ry) to avoid
discontinuities in the potential energy vs. volume curve.  However, when
employing a constant number of grid points, by making use of the reference
cell method, these jumps can be avoided (but in the paper the authors use
also the very high 1200 Ry-cutoff in this method).
Knowing this, I have two questions about this:
i) what is the 'default'-method the generate the grid in CP2K: constant
density of points or constant number of points?
ii) if the default-method is to generate a constant number of points, is it
then also necessary to take such a big cut-off of 1200 Ry? Or is it then
good enough to go a bit smaller? In this paper: J. Phys. Chem. B, 2009,
113, 11959, the authors take 700 Ry for the reference cell method, but why
exactly this value? It is 500 Ry less then the proposed 1200 Ry as it was
proposed in the first reference...

many thanks
marc
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