[CP2K-user] [CP2K:21065] Question about total and molecular dipoles

[Fangyong Yan]

The electron density is the summation of all the Wannier functions squares,
so if an electron density at a cartesian coordinate is 0.1, this is
contributed by
all the wannier functions at this position.
So you can think wannier function is one way of partitioning the electron
density.

FY

On Sun, Jan 19, 2025 at 4:35 PM Fangyong Yan <fangyongyan2023 at gmail.com>
wrote:

> 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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