[Pw_forum] dipole moment calculation

[Giovanni Cantele]

Aihua Zhang wrote:
> Dear all,
>
> I tried a test calculation of the dipole moment of water. The water 
> molecule is put into a 20 a.u. ^3 box. The relevant setting are
>
> lelfield = .true.
> efield = 0.01
>
> In the output file, it reports
>
>      Electronic Dipole per cell (a.u.)   -1.921145549061207
>      Ionic Dipole per cell (a.u.)    116.1997104738884
>
> How is Electronic Dipole defined here? By looking into the source 
> code, Ionic Dipole is defined as in textbook. I also notice that total 
> energy difference with respect to zero-electric-field system is 
> approximately equal to (Electronic Dipole + Ionic Dipole) * Efield. 
> But Electronic Dipole hardly resembles the experimental dipole of 
> water, which is about 1.85 D. Could anyone explain a bit about these 
> quantities? Thank you!!
>
> Zhang
>
>
Hi,

I'm not very expert about this, however I tried to reproduce your job. I 
got:
     Computed dipoles :                               
       electron   0.00000   0.00000  -4.61164         
       ion        0.00000   0.00000  -3.87896         
       total      0.00000   0.00000   0.73268         
        Dipole field [a.u.]:          0.0012

As far as I understand total dipole moment = 0.73268  a.u. = 0.73268 * 
25.417463 / 10 Debye = 1.86 Debye.

I remember I found strange results in electric field calculations some 
time ago, and the problem was where the
molecule is placed within the unit cell, but I'm not very sure (that was 
a couple of years ago!).
The input I used is in attachment.

Giovanni


-- 



Dr. Giovanni Cantele
Coherentia CNR-INFM and Dipartimento di Scienze Fisiche
Universita' di Napoli "Federico II"
Complesso Universitario di Monte S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
Phone: +39 081 676910
Fax:   +39 081 676346
E-mail: giovanni.cantele at cnr.it
        giovanni.cantele at na.infn.it
Web: http://people.na.infn.it/~cantele
Research Group: http://www.nanomat.unina.it

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