The ABINIT Users Mailing List ( CLOSED )

["Artem R. Oganov" <a.oganov@mat.ethz.ch>]

Dear Jorge,

It seems to me that you are using a too small Ecut. Have you tried to
increase it and see what happens? This would probably give you
nearly-zero acoustic frequencies, but also could resolve your problem:
ABINIT always gives some small deviations from the exact degeneracy
(consider this as a numerical error), and I would expect these
deviations to become smaller as you increase Ecut - in other words, in a
crude calculation the degeneracy may be hidden by  the errors due to a
small number of plane waves. If you try your calculation with a bigger
Ecut - please let me know what you see. 

With my best regards,

Artem
************************
Dr. Artem R. Oganov
Lab. of Crystallography
Department of Materials
ETH Zurich
CH-8092 Zurich
Switzerland
Tel.: +41-1-632-3752; Fax: +41-1-632-1133
E-mail: a.oganov@mat.ethz.ch
Website: http://olivine.ethz.ch/~artem
************************ 


-----Original Message-----
From: Jorge Iniguez [mailto:jiniguez@nist.gov] 
Sent: Thursday, August 07, 2003 7:35 PM
To: forum@abinit.org
Subject: [abinit-forum] phonons of hexagonal systems: probable bug

Dear abinitioners,

We have found what seems to be bug in the calculation of phonons of
hexagonal systems. Input and output files go attached.

We have studied MgB2, which has 3 atoms per unit cell and space group
P6_3/mmm.  At the Gamma point, this system should have four different
optical phonon frequencies, two of them being doubly degenerate.  When
we
try to calculate them, abinit correctly recognizes four (as opposed to
nine) irreducible perturbations:

 The list of irreducible perturbations for this q vector is:
    1)  idir= 1 ipert= 1
    2)  idir= 3 ipert= 1
    3)  idir= 1 ipert= 2
    4)  idir= 3 ipert= 2

However, the final result is a list of frequencies...

  Phonon wavevector (reduced coordinates) :  0.00000  0.00000  0.00000
 Phonon energies in Hartree :
   8.954702E-04  1.138484E-03  1.197535E-03  2.116134E-03  3.353738E-03
   3.641500E-03  4.411735E-03  4.837036E-03  5.507493E-03
 Phonon frequencies in cm-1    :
-  1.965330E+02  2.498684E+02  2.628285E+02  4.644378E+02  7.360604E+02
-  7.992168E+02  9.682638E+02  1.061607E+03  1.208755E+03

... that does not show the expected degeneracies!!  I could only briefly
look for the bug, but did not succeed, so I turn to the experts out
there :)

Cheers,
Jorge

PS: I guess that the nonzero acoustic frequencies are to be attributed
to
the problem described in the FAQ...


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   Jorge Iniguez             NIST Center for Neutron Research
                             100 Bureau Drive, Mail Stop 8562
                             Gaithersburg, MD 20899-8562
                             Phone: 1-301-975-8367 Fax: 1-301-921-9847
                             E-mail address: jiniguez@nist.gov
                             http://www.ncnr.nist.gov/staff/jorge
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