[CP2K-user] [CP2K:17133] Re: Energy differences when a molecule with a dipole moment is moved through a PBC

Marcella Iannuzzi marci.akira at gmail.com
Sun Jun 12 10:56:21 UTC 2022

Dear Alex

See section 2.4 in https://doi.org/10.1016/j.cpc.2004.12.014.
Improvements are obtained by applying smoothening (see CP2K_INPUT 
<https://manual.cp2k.org/cp2k-9_1-branch/CP2K_INPUT.html> / FORCE_EVAL 
<https://manual.cp2k.org/cp2k-9_1-branch/CP2K_INPUT/FORCE_EVAL.html> / DFT 
<https://manual.cp2k.org/cp2k-9_1-branch/CP2K_INPUT/FORCE_EVAL/DFT.html> / 
or using GAPW


On Saturday, June 11, 2022 at 10:21:50 PM UTC+2 alexvd... at gmail.com wrote:

> Hi Matt,
> Thanks for the feedback. Which XC functionals are particularly susceptive 
> to this effect? Do you have some references I could look into for this? I 
> tried increasing the PW cutoff, but it is quite costly, and makes it 
> difficult for me to do a high-throughput screening experiment.
> Best,
> Alex
> On Friday, 10 June 2022 at 16:13:41 UTC-4 mattwa... at gmail.com wrote:
>> Hi Alex,
>> this isn't to do with electrostatics as such, I don't think - it is 
>> something called the eggbox effect. CP2K projects the density onto a 
>> regular grid to calculate electrostatics and XC energy and this projection 
>> is not translationally invariant as you move atoms across the cell. The 
>> variations you see are probably due to the effect being cancelled out to 
>> some extent in some cases.
>> The simplest solution is to increase the PW cutoff in the MGRID section 
>> to alleviate the issue. It also depends on the XC functional used.
>> Matt
>> On Friday, 10 June 2022 at 20:16:23 UTC+1 alexvd... at gmail.com wrote:
>>> Sorry, I should clarify the files attached. In Z-align, the CO molecule 
>>> is aligned with the Z axis; Zperp is perpendicular to the Z axis. In the 
>>> *closepack files, the PBC is 1x1x10, whereas in the others it is 5x5x10. 
>>> The results are in the plot folder; where the figure 
>>> Z-align-closepack.txt.png plots the energy difference as a function of Z 
>>> displacement; a variation of 0.12 Hartrees is seen. In all the other 
>>> experiments, the variation is much smaller, 0.001 Hartrees. 
>>> Best,
>>> Alex
>>> On Friday, 10 June 2022 at 15:09:18 UTC-4 alex van der waals wrote:
>>>> Hi,
>>>> I've been seeing a strange effect from a calculation. I have a single 
>>>> CO molecule, that I am displacing through the Z axis of a 5x5x10, 2x2x10 
>>>> and a 1x1x10 PBC (with periodicity on for all axes, angles=90 degrees for 
>>>> all), and doing single point energy calculations for each. I expect that 
>>>> because its periodic, there should be no difference in energy where in the 
>>>> PBC I put the CO molecule. Strangely however, there is an energy 
>>>> difference; as I scan the Z axis, the energy reported fluctuates up and 
>>>> down periodically. I am using the PBE functional  and a cutoff=300 fwiw.
>>>> I have attached figures and input files for the calcultions below.
>>>> Is this a known phenomenon?
>>>> Best,
>>>> Alex

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