[CP2K-user] [CP2K:17128] Re: Energy differences when a molecule with a dipole moment is moved through a PBC

[Matt Watkins]

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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