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

[Miguel Steiner]

Dear Prof. Hutter,

thanks for your response, that obviously makes sense.
I guess this is even more exaggerated, due to the large box size? 
Do you (and your team) have experience what box size fits best to simulate 
a molecular calculation. I need those to calculate adsorption energies.

Thanks and best wishes,
Miguel

jgh schrieb am Mittwoch, 3. März 2021 um 15:06:54 UTC+1:

> 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 <+41%2044%20635%2044%2091>
> Institut für Chemie C FAX : ++41 44 635 6838 <+41%2044%20635%2068%2038>
> Universität Zürich E-mail: h... at chem.uzh.ch
> Winterthurerstrasse 190
> CH-8057 Zürich, Switzerland
> ---------------------------------------------------------------
>
> -----c... at googlegroups.com wrote: -----
> To: "cp2k" <c... at googlegroups.com>
> From: "Miguel Steiner" 
> Sent by: c... 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]
>
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