[CP2K:4719] dbcsr error when applying periodic efield

[David Stelter]

Hi Matt and Flo,

There seem to be some descrepencies in the methods mentioned above. I've 
done a few tests on a single water molecule with multiple versions of 
EXTERNAL_POTENTIAL and PERIODIC_EFIELD as well as no efield.

The energies I obtained for a single water for various inputs are as 
follows with an inputted/ideal e-field strength of 0.1 V/angstrom
1) no field, E =   -15.902943301939017

2) PERIODIC_EFIELD, E =   -15.901968922615296

3) EXTERNAL_POTENTIAL in the FORCE_EVAL section only, E =   
 -15.90294330193902

4) EXTERNAL_POTENTIAL in the DFT section only, E =   -16.227568014660498

5) EXTERNAL_POTENTIAL in both sections, E =   -15.971096979028731

It seems there is another section of EXTERNAL_POTENTIAL that works under 
the FORCE_EVAL section rather than DFT. At first inspection, it seems that 
this section would only apply to the MM region of the cell as there is 
basically no difference between values 1 and 3. However, as you can see, 
there is a large difference between 4 and 5 where EXTERNAL_POTENTIAL is 
applied to both the DFT and FORCE_EVAL sections.

Additionally, it should be noted that PERIODIC_EFIELD is being used with a 
value of 20x10^-4 au in the Y direction, which corresponds to 
~0.1V/angstrom while EXTERNAL_POTENTIAL is being used via the function 
Y*(A/B) with values of 0.1 [eV] and 1 [angstrom] as suggested by Matt to 
obtain a 0.1V/angstrom field. However, as you can clearly see, the energy 
values are VERY different which leads me to believe there is something 
wrong! I am not using ROKS or QM/MM for this sim thus PERIODIC_EFIELD 
should work fine; additionally the manual is very clear and explicit by 
saying the input is in au. This makes me fairly confident in the accuracy 
of this calculation for the energy of 1 QM water in a 0.1V/angstrom field

I am still unsure regarding the units of EXTERNAL_POTENTIAL. eV/Angstrom is 
a force, multiplied by Z, a unit length, brings us back to an energy of 0.1 
eV which is being applied to the system. I guess I am confused regarding 
Matt's calculation of efield strength from 0.1 eV/Angstrom input. Any light 
that can be shed on how the EXTERNAL_POTENTIAL process works and how the 
potential is being applied would be great as I am certain that the input 
for EXTERNAL_POTENTIAL is resulting in a field strength much higher than 
0.1 V/angstrom. 

I've attached my input/output files of the sims. It should also be noted 
that these input files came from the test folder of cp2k and are identical 
except for the various electric field codes.

Thanks,
David

On Monday, October 28, 2013 9:18:29 AM UTC-4, Matt W wrote:
>
> Hi David,
>
> On Friday, October 25, 2013 9:03:09 PM UTC+1, David Stelter wrote:
>>
>> Hi Matt,
>>
>> We have looked into EXTERNAL_POTENTIAL, some guidance on the section 
>> would be appreciated! I've read on these forums that people have put an 
>> efield value as a parameter in this section. How is this section treated? I 
>> want to apply an efield, but shouldn't I be putting in a potential 
>> multiplied by some distance of the periodic box?
>>
>>
> in the DFT section
>
> &EXTERNAL_POTENTIAL
>     FUNCTION Z*(A/B)
>     PARAMETERS A B
>     VALUES [eV] 0.1 [angstrom] 1.0
> &END
>
> should add a field of 0.1 V/A along the Z direction of your QM cell. This 
> is just a classic sawtooth potential - but as your QM region has vacuum 
> around it there should be no issues.
>
> As Flo notes for the periodic_efield, I think this will only act on the QM 
> region. Adding a similar potential over the classical region would cause 
> issues with periodicity. 
>
> I haven't tried it in combination with ROKS or QMMM.
>
> Maybe an easier solution is just to add some artificial charges into the 
> MM region to generate the field? Though you'd have to take into account 
> that their effect would get screened by solvent...
>
> Matt
>
>
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