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Re: Problem with Coulomb Energy

From: Georg Rempfer
Subject: Re: Problem with Coulomb Energy
Date: Wed, 13 Nov 2019 20:59:18 +0100

I am not sure whether that is relevant in this situation, but periodic boundary conditions can have the effect of reducing potential differences by a factor of two.

On Wed, Nov 13, 2019 at 4:16 PM Fabian Glatzel <address@hidden> wrote:
Dear developers,

using your package I encountered some problem I could not resolve by myself. I'd be deeply grateful if you could offer some advice.

The problem is the following:
Using P3M and ELC with an external potential (via the "pot_diff" argument), I get Coulomb energies that make no sense. The energy contribution of charges in the "external" potential defined by "pot_diff" seems to be off by a factor of 2.
Let's say the system has a length of 10 and pot_diff=1V is set. Further, only two particles are placed in the system at z=0 (charge q=+1e) and at z=9 (charge q=-1e) and the prefactor for the P3M is set such that the interactions among the particles are negligible. For this case, I would expect a Coulomb energy of approx. 0.8eV. However, espresso seems to have a different opinion, namely that it should be 0.4eV.

Please find a minimal example code attached to this email.

I'm looking forward to hearing from you,

Fabian Glatzel

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