# [CP2K-user] Dipole Moment. Help wanted.

Alexander Kazakov alexande... at gmail.com
Fri Jun 25 13:58:39 UTC 2021

```Yeap. Topology is here.

Best regards,
Alexander Kazakov

пятница, 25 июня 2021 г. в 15:55:24 UTC+2, Alexander Kazakov:

> Dear CP2K community,
>
> I am simulating 64 molecules of water using DFT in NVT ensemble using PBC.
> I interested in dipole moment values as a trajectory with MD step.
>
> Here is what I have so far:
> [image: DM_xyz_total.png]
>
> You can see 4 curves: 3 of them are components (x,y,z) and the last one is
> the length of the vector.  All of them are functions of the MD step.
>
> Let's focus on curves that correspond to x and z components. You can see
> that there is a kind of "jump"  at certain ranges. These "jump" actually
> resemble a crossing kind of boundary condition. You can see how well it is
> fit the blue curve on the top to the bottom (the same is applied for the
> purple curve).
> Personally, I think it is indeed a kind of boundary condition, however, I
> have no idea what 30 means.
>
> Then I found these certain steps where it happens:
>
> MD STEP
> 26
>     X=  -19.18787840 Y=    5.85693184 Z=   29.55510907     Total=
> 35.72090144
> 27
>     X=  -19.18076162 Y=    5.52351367 Z=  -29.53176935     Total=
> 35.64458192
>
> So the z component apparently changed the sign. So I thought that
> something horrible happened in the system. I plotted position atom change
> as a function of atom ID:
> [image: 26-27_diff.png]
>
> Alright, there is an interesting plot here. We see the different amplitude
> of change through the atom IDs. So first 2/3 of the curve correspond to
> hydrogen atoms displacement, whereas the last part corresponds to oxygen
> atoms.
>
> *So back to questions.* Could you tell me, why some components of the
> dipole moment "jump" changing the sigh? Is it possible to force dipole
> moment to stay on one side of the "boundary"? There are no doubts that
> hydrogen atoms should move "actively" than oxygen atoms, but I don't
> understand what has changed dramatically in the system. Lastly, if you can
> give me a hint for number 30 in the boundary (see figure), I would be
> thankful!
>
> As because I check CP2K 6.1, 7.1, 8.1, and 8.2 and I got the same
> behaviour with jumps I assume that I did something wrong or misunderstand
> something.
> Please find enclosed FORCE_EVAL section of my script.
>
> Additionally I am sending topology of the system.
>
>
> Best regards,
> Alexander Kazakov
>
>
>
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