[CP2K-user] [CP2K:21064] Question about total and molecular dipoles

[Fangyong Yan]

The Wannier center does not mean the electron is there, because electron is
only known by a probability and electrons are indistinguishable.
You can think the dipole moment operator is Wannier center multiplying the
electron charge.

For non-periodic electrons, there should be methods just like Wannier
functions for periodic electrons.

FY

On Sun, Jan 19, 2025 at 4:24 PM Fangyong Yan <fangyongyan2023 at gmail.com>
wrote:

> Dear Hana,
>
> Wannier function is only for periodic systems. (see for example, our
> paper: Infrared Spectroscopy of Li+ Solvation in EmimBF4 and in Propylene
> Carbonate: Ab Initio Molecular Dynamics and Experiment, JPCB, 2022)
> The wannier center is treated as the expectation value of the position of
> individual electrons. That is why we can obtain molecules, or cation
> with +1 charge, and anion with -1 charge. Because
> all the charges (electrons, nuclei), are integer.
> Wannier function is one way to partition the electron density, there are
> other methods to partition the charge density.
>
> For a molecule, the dipole moment is invariant to your chosen origin. For
> a cation or anion, dipole is dependent on your origin, but the change of
> dipole is not, and IR intensity is related to the change of dipole.
>
> Good luck!!
>
> Fangyong
>
>
> On Sat, Jan 18, 2025 at 12:40 PM Hana <holiaei2 at illinois.edu> wrote:
>
>> Hello,
>>
>> I am trying to calculate the dipole moment of my non-periodic system. The
>> system has more than one molecule, and I am interested in calculating both
>> molecular and total dipole moment. Currently, I use two sections (section1:
>> FORCE_EVAL/DFT/LOCALIZE, section 2: FORCE_EVAL/DFT/PRINT/MOMENTS) in my
>> input, as elaborated at the end of this message. Section 2 outputs a total
>> dipole moment of 18.9465 Debye. From section 1 and using the Wannier
>> centers, I get a total dipole moment of 18.9876 Debye. Here is my first
>> question: Is this discrepancy normal? According to CP2K documentation, the
>> first value (from section 2) is based on "simple operator". However, I am
>> not sure what that is, and whether it justifies the discrepancy?
>>
>> Also, I have another question. In section 1, I have the TOTAL_DIPOLE and
>> MOLECULAR_DIPOLES subsections as well. The dipole values that I get from
>> these two are completely (orders of magnitude) different from the values
>> mentioned in the previous paragraph (~19 Debye), although I set all the
>> reference points to zero. Moreover, for the molecular dipole, the output
>> file shows zero for most of the atoms, and non-zero values for the rest of
>> them. Therefore, I cannot find the "molecule" concept there. Here is my
>> second question: Am I missing anything when using these two subsections
>> (TOTAL_DIPOLE and MOLECULAR_DIPOLES)?
>>
>> Thanks in advance for your help, and sorry for the long message!
>>
>> *section 1:*
>>     &LOCALIZE
>>       MAX_ITER 1000
>>       METHOD JACOBI
>>       EPS_LOCALIZATION 1.0E-6
>>       &PRINT
>>         &WANNIER_CENTERS
>>            IONS+CENTERS .TRUE.
>>         &END
>>         &TOTAL_DIPOLE
>>            PERIODIC F
>>            REFERENCE ZERO
>>            FILENAME total
>>         &END
>>         &MOLECULAR_DIPOLES
>>            PERIODIC F
>>            REFERENCE ZERO
>>            FILENAME molecular
>>         &END
>>       &END
>>     &END
>>
>> *section 2:*
>>     &PRINT
>>       &MOMENTS
>>             PERIODIC F
>>             REFERENCE ZERO
>>             FILENAME dipole
>>       &END
>>     &END
>>
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>>
>

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