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

Dear Rongqin,

Your results may actually be correct! Phonon softening can indeed occur
at the Gamma point. You have large imaginary frequencies, which tell
that the structure is unstable. To understand the instability better, I
suggest you plot phonon dispersion curves along the most important
directions in your structure. This may reveal the most unstable
q-vectors. 
Born charges in some situations can exceed the formal valence even by a
factor of 2-3! For an explanation and some examples see: Ghosez Ph.,
Michenaud J.-P., Gonze X. (1998). Dynamical atomic charges: the case of
ABO3 compounds. Phys. Rev. B58, 6224-6240. This means, if you like, that
these charges do not rigorously correspond to the traditional chemical
concepts of bond ionicity. 

With my best regards,

Artem
++++++ESF/CECAM/Psi-k Workshop: http://www.wrc.man.ac.uk/cecam/
************************ 
Dr. Artem R. Oganov 
Lab. of Crystallography, Department of Materials 
ETH-Honggerberg, Wolfgang Pauli Strasse 10, 
CH-8093 Zurich, Switzerland
Tel.: +41-1-632-3752; Fax: +41-1-632-1133
E-mail: a.oganov@mat.ethz.ch 
Personal web-page: http://olivine.ethz.ch/~artem
Group page:  http://www.crystal.mat.ethz.ch/research/ExtremeConditions
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-----Original Message-----
From: Wu Rongqin [mailto:g0203654@nus.edu.sg] 
Sent: 18 July 2004 17:07
To: forum@abinit.org
Subject: [abinit-forum] about negative phonon frequency at gamma!

Dear all,
I use abinit to calculate dielectric properties of cubic perovskite
SrTiO3 which is only stable at high temperature. 

A,The result showed negative values at gamma point. 
Phonon at Gamma, with non-analyticity in the
  direction (cartesian coordinates)  0.00000  0.00000  1.00000
 Phonon energies in Hartree :
  -4.486115E-04 -4.486105E-04 -2.927791E-05 -2.869101E-05 -2.868774E-05
   2.908060E-04  2.908060E-04  2.908060E-04  5.257838E-04  8.833161E-04
   8.833162E-04  1.555478E-03  2.653876E-03  2.653876E-03  3.533046E-03
 Phonon frequencies in cm-1    :
- -9.845885E+01 -9.845862E+01 -6.425758E+00 -6.296948E+00 -6.296231E+00
-  6.382455E+01  6.382455E+01  6.382455E+01  1.153962E+02  1.938655E+02
-  1.938655E+02  3.413880E+02  5.824585E+02  5.824585E+02  7.754140E+02
I have check the k point and energy cutoff. Is this result normal? Can
softening happen at gamma point?
2, the effective charges seemed also unreasonable
1    7   1    3   -1.8723143702    0.0000000000
   2    7   1    3    0.0000000000    0.0000000000
   3    7   1    3    0.0000000000    0.0000000000

   1    7   2    3    0.0000000000    0.0000000000
   2    7   2    3   -4.8593631119    0.0000000000
   3    7   2    3    0.0000000000    0.0000000000

   1    7   3    3    0.0000000001    0.0000000000
   2    7   3    3    0.0000000000    0.0000000000
   3    7   3    3   -1.8723135534    0.0000000000

   1    7   1    4   -1.8723135534    0.0000000000
   2    7   1    4    0.0000000000    0.0000000000
   3    7   1    4    0.0000000000    0.0000000000

   1    7   2    4    0.0000000001    0.0000000000
   2    7   2    4   -1.8723135534    0.0000000000
   3    7   2    4    0.0000000000    0.0000000000

   1    7   3    4   -0.0000000001    0.0000000000
   2    7   3    4    0.0000000000    0.0000000000
   3    7   3    4   -4.8593631119    0.0000000000

   1    7   1    5   -4.8593631119    0.0000000000
the -4.8593631119 is for oxygen atom. Which should be less than -2!

Any comment will be appreciated!
Rongqin Wu