[QE-users] Dipole moment of the CO2 molecule

[Giovanni Cantele]

An issue with your input is that you place the point where the external saw-tooth potential
has a discontinuity in its derivative at emaxpos = 0.5 (it is in crystal units, so because edir = 3,
it means that the potential depends on z and the discontinuity is at (0,0,a/2)).
However, the CO2 molecule is placed along the x axis at fixed y = a/2 and z = a/2. As such,
the discontinuity, which is unphysical since in serves to compensate the spurious dipole field that
would arise as a consequence of the periodic boundary conditions, is located in a region where the
charge density is not zero. emaxpos should be set in such a way that the discontinuity il located
in the vacuum, in the region where the charge density is zero or in any case very small. In your setup
it seems that a good choice would be emaxpos=0.

Giovanni

PS since you’re studying an isolated molecule, provided the size of your supercell is sufficiently large,
the eigenvalues should exhibit no dependence on k. As a consequence, in this case, a 4x4x4 sampling
of the Brillouin zone, should provide results equivalent to a gamma only sampling. So, you can switch to
KPOINTS automatic
1 1 1    0 0 0
or, even better,
KPOINTS gamma
to make your calculation faster while keeping the same accuracy.

> On 27 May 2020, at 03:42, ENDALE ABEBE <endtse at gmail.com> wrote:
> 
> Dear Experts, users and all
> 
> I found the dipoles of CO2 calculated by Quantum ESPRESSO as :
> 
> Computed dipole along edir(3) :
> Elec. dipole 0.3112 Ry au, 0.7909 Debye
> Ion. dipole 0.8137 Ry au, 2.0683 Debye
> Dipole 41.9812 Ry au, 106.7055 Debye
> Dipole field 0.5025 Ry au, 
> 
> I assumed the third value is the sum of the electronic and ionic contributions.
> The input and output files are attached herewith.
> Since CO2 is non-polar molecule (with polar bonds), shouldn't the total dipole moment be zero?
> 
> -- 
> Endale Abebe
> Program coordinator and Lecturer
> Faculty of Materials Science and Engineering
> Jimma Institute of Technology
> Jimma University
> P.O.Box 378, Jimma, Ethiopia
> Mobile: +251921381598
> <CO2_nscf.out><CO2_nscf.in>_______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu/quantum-espresso)
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-- 

Giovanni Cantele, PhD

CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy

e-mail: giovanni.cantele at spin.cnr.it
            gcantele at gmail.com
Phone: +39 081 676910
Skype contact: giocan74
Web page: https://sites.google.com/view/giovanni-cantele

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