[gpaw-users] Boundary conditions for poisson solver different for grid and pw mode?

[Filip Anselm Rasmussen]

Dear GPAW'ers

I am calculating the vacuum level of graphene and noticed that it varied 
with ~0.3 eV depending on whether I use FD or PW mode. The calculations 
seem to be very well comverged, so I plotted the in-plane averaged 
effective potential along the z-direction (see attached figures) and 
noted that even though I use periodic boundary conditions (even in the 
z-direction where I have only one k-point), the slope of the FD 
effective potential at the boundaries is not zero as I expected. In PW 
mode the slope seems to be zero.

So my question is: Does FD mode use non-periodic boundary conditions if 
there is only one k-point along some axis - even though you have 
pbc=True for your Atoms object?

Input scripts have been attached. I calculate the effective potential 
along z like this:

Veff_z = np.mean(np.mean(calc.get_effective_potential(), axis=0), axis=0)


Thanks,
Filip Rasmussen
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