[CP2K-user] [CP2K:14861] Energy differences with identical relative coordinates

[hut... at chem.uzh.ch]

Hi

due to the integration grid (PW grid) the QS energy is not
translationally invariant. When you move a molecule rigidly
through the box, the energy will oscillate. The frequency and
amplitude depend on you PW cutoff.

best regards

Juerg Hutter
--------------------------------------------------------------
Juerg Hutter                         Phone : ++41 44 635 4491
Institut für Chemie C                FAX   : ++41 44 635 6838
Universität Zürich                   E-mail: hut... at chem.uzh.ch
Winterthurerstrasse 190
CH-8057 Zürich, Switzerland
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-----cp... at googlegroups.com wrote: -----
To: "cp2k" <cp... at googlegroups.com>
From: "Miguel Steiner" 
Sent by: cp... at googlegroups.com
Date: 03/03/2021 01:49PM
Subject: [CP2K:14861] Energy differences with identical relative coordinates

Dear Cp2k developers,

just by coincidence I noticed that I get different electronic energies depending on the absolute coordinates of my input molecule.
I tried a simple test system with O2 in a 15x15x15x angstrom cube. I set-up 2 different xyz files, one with 3.xx angstrom in the z coordinates and one with 34.xx angstrom in the z coordinate. However, both O2 molecules are identical in their bond length, they are simply translated to each other.
As long as I center the coordinates, I get the identical electronic energy of
-31.929620495691506
But if I don't center the coordinates, I get different results:
-31.929366281248029 (inside)
-31.929298672694223 (outside)

I mean, I can imagine that it leads to difficulties, if the molecule is placed outside of the defined cell, but especially the different energy for the O2 molecules that is placed within the cell, albeit not the exact center, confuses me.

Am I doing something wrong here or is this expected behavior? If the latter is the case, why aren't all coordinates centered by default (which I guess I will do from now on)?

Input and systems see attachment.

Thanks in advance and best regards,
Miguel
  
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[attachment "center-outside.inp" removed by Jürg Hutter/at/UZH]
[attachment "system-outside.xyz" removed by Jürg Hutter/at/UZH]
[attachment "center-inside.inp" removed by Jürg Hutter/at/UZH]
[attachment "system-inside.xyz" removed by Jürg Hutter/at/UZH]
[attachment "nocenter-inside.inp" removed by Jürg Hutter/at/UZH]
[attachment "nocenter-outside.inp" removed by Jürg Hutter/at/UZH]