[CP2K:6441] Problems with GAPW + external potential

[Matt W]

OK, that patch is in the trunk now.

Matt

On Monday, May 25, 2015 at 10:42:21 AM UTC+1, Juha Ritala wrote:
>
> I have tested the patch using the water molecule setup and everything 
> looks good so far. The total energy stays almost perfectly constant (up to 
> a negligible numerical error) when a constant background potential is 
> applied. This was more or less obvious so I continued by applying a uniform 
> electric field to the system (maximum magnitude 0.1 V/Å). The dependence of 
> the energy on the electric field is the same (within the eps_scf) using 
> both GPW and GAPW methods. Also the forces on the atoms are almost the 
> same. The fixed GAPW method does a better job here, actually, since the sum 
> of the forces is closer to zero, which should be the case for the charge 
> neutral molecule.
>
> Continuing the theoretical consideration, I would say that this fix should 
> handle an uniform electric field exactly independently on the magnitude of 
> the field. This requires only that the monopole and dipole moments of the 
> charge density in the atomic region are correct, which should be the case 
> with *rho0_s_rs* (*ñ^0).* More generally, any external potential that 
> corresponds to a charge density outside the atoms should be fine, I guess. 
> This would be true because outside the atomic region the electrostatic 
> potential generated by the atomic charge density depends only on the 
> multipole moments of the density. Since those are correct, everything 
> should work perfectly.
>
> I will next try a real simulation setup where the field is not perfectly 
> uniform.
>
> - Juha
>
> On Wednesday, 20 May 2015 11:34:22 UTC+3, Juha Ritala wrote:
>>
>> Hi Matt,
>>
>> thank you for the patch. I think this approach makes sense. If I 
>> understood correctly, *rho0_s_rs* is the realspace grid representation 
>> of the charge density *ñ^0* (in the GAPW paper from 1999), and *ñ^0 *is 
>> sort of a representation of the charge density in the atomic region using 
>> the set of soft Gaussians. At least the multipole moments of the atomic 
>> charge density and *ñ^0 *are equal, so in a sufficiently large length 
>> scale these charge densities are approximately the same. This would imply 
>> that for moderate electric fields this approximation would indeed be fine.
>>
>> I will check how this patch works for me and keep you informed.
>>
>> - Juha
>>
>> On Saturday, 16 May 2015 13:03:25 UTC+3, Matt W wrote:
>>>
>>> Hi Juha,
>>>
>>> there are a couple of files attached (compatible with latest version of 
>>> cp2k at time of writing, but there are only a couple of lines changed) you 
>>> can try and see if it works with GAPW. It only works with the soft part of 
>>> the density, including the compensation charges. As the compensation 
>>> charges, including the core, are put onto the fft grid the core routines in 
>>> external_potential need to be protected. Might need tidying for mixed 
>>> gpw/gapw.
>>>
>>> I guess this approximately means that the core (i.e. any functions on 
>>> the hard atomic grids) isn't directly polarized by the field? For modest 
>>> fields it should be fine, maybe?
>>>
>>> Let me know how it goes,
>>>
>>> Matt
>>>
>>>
>>> On Friday, May 1, 2015 at 3:46:30 PM UTC+1, jgh wrote:
>>>>
>>>> Hi 
>>>>
>>>> unfortunately, I don't have time to look into this. 
>>>> It seems to me that for analytic potentials it wouldn't be that hard, 
>>>> but for potentials read in on a grid it would need special care. 
>>>>
>>>> regards 
>>>>
>>>> Juerg 
>>>> -------------------------------------------------------------- 
>>>> Juerg Hutter                         Phone : ++41 44 635 4491 
>>>> Institut für Chemie C                FAX   : ++41 44 635 6838 
>>>> Universität Zürich                   E-mail: hut... at chem.uzh.ch 
>>>> Winterthurerstrasse 190 
>>>> CH-8057 Zürich, Switzerland 
>>>> --------------------------------------------------------------- 
>>>>
>>>> -----cp... at googlegroups.com wrote: -----To: cp... at googlegroups.com 
>>>> From: Juha Ritala 
>>>> Sent by: cp... at googlegroups.com 
>>>> Date: 04/30/2015 01:18PM 
>>>> Subject: Re: [CP2K:6441] Problems with GAPW + external potential 
>>>>
>>>> Hi Juerg, 
>>>>
>>>> thank you for confirming this. Is there any chance that this will be 
>>>> fixed in the near future, or should I stick with GPW for the time being? I 
>>>> could try and fix it myself, but as far as I understand how GAPW works, 
>>>> considering the contribution from the local atomic charges is not at all 
>>>> trivial. 
>>>>
>>>> - Juha 
>>>>
>>>> On Wednesday, April 29, 2015 at 2:17:45 PM UTC+3, jgh wrote:Hi 
>>>>
>>>>
>>>>
>>>> yes, this is a bug. The external potential is applied to the 
>>>>
>>>> charge on the plane wave grid. In GAPW only the soft part 
>>>>
>>>> of the charge is on the grid and one would have to also compute 
>>>>
>>>> the contributions from the local atomic charges. 
>>>>
>>>>
>>>>
>>>> regards 
>>>>
>>>>
>>>>
>>>> Juerg Hutter 
>>>>
>>>> -------------------------------------------------------------- 
>>>>
>>>> Juerg Hutter                         Phone : ++41 44 635 4491 
>>>>
>>>> Institut für Chemie C                FAX   : ++41 44 635 6838 
>>>>
>>>> Universität Zürich                   E-mail: hut... at chem.uzh.ch 
>>>>
>>>> Winterthurerstrasse 190 
>>>>
>>>> CH-8057 Zürich, Switzerland 
>>>>
>>>> --------------------------------------------------------------- 
>>>>
>>>>
>>>>
>>>> -----cp... at googlegroups.com wrote: -----To: cp... at googlegroups.com 
>>>>
>>>> From: Juha Ritala 
>>>>
>>>> Sent by: cp... at googlegroups.com 
>>>>
>>>> Date: 04/29/2015 09:52AM 
>>>>
>>>> Subject: [CP2K:6432] Problems with GAPW + external potential 
>>>>
>>>>
>>>>
>>>> I have had some problems when I tried to use GAPW method combined with 
>>>> an external potential. To demonstrate what seems to be the source of the 
>>>> problems, I made this simple test case with a water molecule in a constant 
>>>> external potential. Physically, the constant background potential should 
>>>> not affect the total energy of the water molecule since the molecule is 
>>>> charge neutral. The energy does change as a function of the constant 
>>>> potential when the GAPW method is used, however. You can see this in the 
>>>> attached energy vs. background potential plot. GPW method gives expected 
>>>> behaviour, the total energy stays perfectly constant when the potential is 
>>>> varied. 
>>>>
>>>>
>>>>
>>>> It seems as if some part of the charge in the system is unaffected by 
>>>> the external potential in GAPW and thus the total charge is effectively 
>>>> non-zero. Is this a bug or is there some part of the implementation 
>>>> missing? 
>>>>
>>>>
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