# [Pw_forum] dipole moment calculation

Aihua Zhang zah7903 at gmail.com
Wed Mar 11 05:24:38 CET 2009

Dear Giovanni and Paolo,

Thank you very much for kind replies. I have reproduced Giovanni's result
with tefield option and with the water molecule at origin. When the molecule
is away from origin, weird results are produced. I look through the code in
add_efield.f90 and compute_dip.f90. I think I found some possible bugs in
them. The first one is not vital. In add_efield.f90, the variable, first, is
set to false at the very begin of the subroutine. But this variable is used
afterwards with an obvious intention that it should be true for the first
time when the subroutine is called. With this small bug, the output of the
sawlike potential setup is missing if the varialble, dipfield, is also true.
The second possible bug is in compute_dip.f90, where r and r in rho(r) seem
dismatch when calculating the electronic dipole. After matching r with
rho(r) in the code, the reasonable identical results are obtains, i.e. the
calculated dipole and total energy are independent of where the molecule is
put in the supercell.

I have only limited understanding of MTP. If I understand correctly, it is
emphasized that the convectional definition of polarization (P=\int r rho(r)
d^3r) is ill-behaved for periodic system and only the change of P is
meaningful. So is it a proper way to calculate static dipols with lberry and
lelfield? BTW, what is the difference and relationship between lberry and
lelfield, both dealing with MTP? Since the tefield approach is rather
straighforward to me and produce reasonable result, I would stick to it for
the time being.

Best regards

Aihua

On Wed, Mar 11, 2009 at 3:28 AM, <umari at democritos.it> wrote:

> Dear Zhang,
>
>
> > 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
>
>
> The option lelfield turns on the calculation of the electronic and
> ionic dipoles under the presence of an electric field.
> While the electronic dipole is calculated through the modern theory of
> the polarization formalism (MTP) which is valid also for
> periodic systems, the reported ionic dipole is given using the simple
> textbook formula. This is correct if you want to calculate  dipole
> differences (e.g. after a structural relaxation or after  the application
> of an electric field) but does not give directly the dipole of an
> isolated molecule. For calculating this, the ionic dipole should be
> calculated consistently through a MTP-like
> formalism:
> e.g.
>
>  zeta_tot=0.d0
>  DO na=1,nat
>      zeta_tot=zeta_tot+(0.d0,1.d0)*tau(pdir,na)*tpi*zv(ityp(na))
>  END DO
>  pola_ion=dimag(log(exp(zeta_tot)))
>  pola_ion=pola_ion*dsqrt(2.d0)/tpiba*dkfact
>  write(stdout,*) "    Ionic Dipole per cell (a.u.)",pola_ion
>
> Note that Ry atomic units are used and to compare with Hartree atomic units
> you should divide by a factor sqrt(2)
>
> Best regards,
>
> Paolo Umari CNR-INFM DEMOCRITOS
>
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