[CP2K-user] [CP2K:17131] Re: Energy differences when a molecule with a dipole moment is moved through a PBC

[alex van der waals]

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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