[Pw_forum] Bug (or not) with epsil = .false. in PH (trunk version)

[Samuel Poncé]

Dear all,

I found out that it was impossible de make a phonon calculation without
calculating the Born effective charge in a semi-conductor even if we set
epsil = .false.

This is due to the routine prepare_q.f90 that is called inside do_phonon.f90
In the routine there is
     IF ( lgamma ) THEN
        !
        IF ( .NOT. lgauss ) THEN
           !
           ! ... in the case of an insulator at q=0 one has to calculate
           ! ... the dielectric constant and the Born eff. charges
           ! ... the other flags depend on input
           !
           epsil = .TRUE.
           zeu = .TRUE.
           zue = .TRUE.


This means that if we compute q=Gamma and if we do not have
gaussian-smearing (i.e. an semi-cond or insulator), then epsil is
automatically set to TRUE.

I know that physically one should have such LO/TO splitting but the user
should be able to choose.

Maybe this forcing could be reported in the input variable ? or simply put
the default value to .TRUE. instead of false and not enforcing that rule?

What do you think?

Best,

Samuel Ponce,
Department of Materials, University of Oxford
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