[CP2K-user] GAPW convergence for Cu and Ni

[Maxime Van den Bossche]

Dear all,

I would like to use the GAPW method in CP2K to study
(late) transition metal clusters, as it would offer
quite an increase in speed compared to GPW (because
GAPW requires less high plane-wave cutoffs for dealing
with such "harder" pseudopotentials).

I had no problems performing GAPW runs for fifth- and 
sixth-row elements (Pd, Ag, Pt, Au), but for the 
fourth-row ones (Ni, Cu), I ran into severe SCF 
convergence problems (strongly oscillatory behaviour).

I managed to create a minimal example, involving just
the Cu dimer, where this behaviour is also present 
(see attached input and output files). In this particular 
case, the SCF does converge when using the DIRECT_P_MIXING
mixing scheme, but this solution is not applicable to 
my original problem, where I need e.g. Broyden mixing.

I then found that lowering the HARD_EXP_RADIUS parameter
to 1.2 Bohr radii or below (compared to the default value
of 1.512) makes the SCF cycle converge without problems.
I realize that the GAPW mode is still a bit more "experimental"
compared to GPW, so I'm fine with having to tinker with
such parameters, but I wanted to check with you:

Does it indeed make sense to you that the default value for 
HARD_EXP_RADIUS is not appropriate here? Would this e.g. 
be related to the shorter 3d-orbital radius for Cu compared to 
Ag-4d and Au-5d?

Best regards,
Maxime
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