[CP2K-user] [CP2K:18720] Re: Calculation of the static dielectric constant of a 3D periodic water system

[Nathalie Smith]

Dear CP2K users,

I hope it's okay to bring up my question again. I tried to set up a 
simulation of bulk water (20³ angstrom³, 267 water molecules, PBCs in all 
directions, spc/e water model) in Lammps and got an expected value of the 
dielectric constant of around 68 when omitting the 3*pi factor in the 
formula of my python script (which one should omit when using SI units, as 
I now know after doing more research). However, omitting this factor makes 
my values that I obtained using CP2K even worse. But now I at least know 
that in principle, my formula is correct. When using CP2K with the same box 
size, amount of water molecules, PBCs, and either the SPC/E model or 
various DFTB parametrizations, I got values around 8/(3*pi).

Could someone help me with this problem, please? I would love to set up 
simulations using DFTB but in the long run, I can't switch to DFTB+ because 
of some missing features, which is why I would like to use CP2K.

Currently, I am using the CP2K version 7.1.

Best wishes,

Nathalie Smith

Nathalie Smith schrieb am Freitag, 14. April 2023 um 14:04:16 UTC+2:

> Dear CP2K users,
>
> I am having trouble with a problem that I have seen sporadically in this 
> forum, but didn't really find an answer to.
>
> I am trying to calculate the static dielectric constant of water using the 
> variance of the total dipole moment of the system, like shown in several 
> papers and CP2K tutorials (e.g. 
> https://www.cp2k.org/exercises:2014_ethz_mmm:monte_carlo_ice or 
> https://www.cp2k.org/exercises:2018_uzh_acpc2:mol_sol). Currently, I am 
> tyring to reproduce the MD simulation shown in the last link, just for 
> longer simulation times (in the nanosecond scale) and in the NVT instead of 
> the NVE ensemble. Therefore, I use the TIP3/Fw water model, PBCs in all 
> directions, the spme method to deal with electrostatic interactions, and 
> the berry phase formula for the calculation of the dipole moments. You can 
> see all the parameters in the attached input file.
>
> When I got the file with the dipole moments, I used the linux terminal 
> command „cat filename.dat | grep Debye > newfilename.dat“ to extract the 
> dipole moments in Debye, removed the „MM DIPOLE [BERRY PHASE](Debye)|“ 
> section and used a python skript to calculate the static dielectric 
> constant from it (also attached to this message). The problem: The value I 
> get is way too low, around 15 and not around 80 as I would expect. In other 
> simulations that I set up (using SPC/E water and a larger cell), the 
> problem was even worse and I got a value around 8. 
>
> I checked my units and formulas several times and I honestly don’t think 
> they are the reason I get wrong values. From what I have seen in the CP2K 
> forum, this is a rather common problem; e.g. here 
> https://lists.cp2k.org/archives/cp2k-user/2020-March/013043.html someone 
> only got the right result after calculating the total dipole moment by 
> themselves (which is something I could do for most MM methods, but not 
> quite as easily for QM methods I guess, which I want to use later on. I 
> know I could calculate molecular dipole moments using maximally localized 
> Wannier centers, but this seems to be computationally more expensive). 
>
> So my question is: Is there something wrong with my calculations that I 
> don’t see? Or is it a problem with the CP2K dipole moments (e.g. wrong 
> unit, or wrong values altogether)? I am convinced that CP2K can be used for 
> the calculation of static dielectric constants using the variance of the 
> total dipole moment because it was already used for this purpose in the 
> literature (e.g. https://pubs.acs.org/doi/10.1021/jp4103355; please note 
> that I am aware of the differences between their and my calculations (MC 
> instead of MD, ice instead of water, DFT instead of a MM water model etc.)).
>
> Thanks in advance for anyone who reads my question, and to anyone who also 
> gives an answer.
>
> Best wishes,
>
> Nathalie Smith
>

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