[CP2K-user] [CP2K:14897] Energy differences with identical relative coordinates

[Thomas Kühne]

Dear Miguel, 

the „egg-box“ you are observing is mainly a matter of the PW cutoff, which by default is only 280 Ry, 
but also due to the G-space differentiation within the XC-potential. Instead of the latter, this can be 
ameliorated by the use of nearest neighbor smoothing and smoothed finite difference operators, e.g. 

     &XC_GRID
         XC_SMOOTH_RHO NN50
         XC_DERIV NN50_SMOOTH
      &END XC_GRID
Cheers, 
Thomas

> Am 08.03.2021 um 13:41 schrieb Miguel Steiner <stein... at gmail.com>:
> 
> 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 <tel:+41%2044%20635%2044%2091> 
> Institut für Chemie C FAX : ++41 44 635 6838 <tel:+41%2044%20635%2068%2038> 
> Universität Zürich E-mail: h... at chem.uzh.ch <applewebdata://E5E00A1A-1EE1-4BAF-9C87-93C9F6AD8E73> 
> Winterthurerstrasse 190 
> CH-8057 Zürich, Switzerland 
> --------------------------------------------------------------- 
> 
> -----c... at googlegroups.com <applewebdata://E5E00A1A-1EE1-4BAF-9C87-93C9F6AD8E73> wrote: ----- 
> To: "cp2k" <c... at googlegroups.com <applewebdata://E5E00A1A-1EE1-4BAF-9C87-93C9F6AD8E73>> 
> From: "Miguel Steiner" 
> Sent by: c... at googlegroups.com <applewebdata://E5E00A1A-1EE1-4BAF-9C87-93C9F6AD8E73> 
> 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 <http://system-outside.xyz/>" removed by Jürg Hutter/at/UZH] 
> [attachment "center-inside.inp" removed by Jürg Hutter/at/UZH] 
> [attachment "system-inside.xyz <http://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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==============================
Thomas D. Kühne
Dynamics of Condensed Matter
Chair of Theoretical Chemistry
University of Paderborn
Warburger Str. 100
D-33098 Paderborn
Germany
tdku... at mail.upb.de
+49/(0)5251/60-5726

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