Problems with GAPW + external potential

[Juha Ritala]

I have had some problems when I tried to use GAPW method combined with an 
external potential. To demonstrate what seems to be the source of the 
problems, I made this simple test case with a water molecule in a constant 
external potential. Physically, the constant background potential should 
not affect the total energy of the water molecule since the molecule is 
charge neutral. The energy does change as a function of the constant 
potential when the GAPW method is used, however. You can see this in the 
attached energy vs. background potential plot. GPW method gives expected 
behaviour, the total energy stays perfectly constant when the potential is 
varied.

It seems as if some part of the charge in the system is unaffected by the 
external potential in GAPW and thus the total charge is effectively 
non-zero. Is this a bug or is there some part of the implementation missing?
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