ABINIT Discussion Forums

Dear Colleagues, we are trying to get frequency at gamma point for S2P2S6 type structure having space group P2_1/c (#14) .
We are getting the frequencies at gamma point (first 10 modes) as below:

Phonon wavevector (reduced coordinates) : 0.00000 0.00000 0.00000
Phonon frequencies in cm-1:
- -1.556207E+00 6.025291E-01 1.787715E+00 1.230568E+01 2.770532E+01
- 4.101165E+01 4.388568E+01 5.063278E+01 5.872738E+01 5.890038E+01
.................................................................

Phonon at Gamma, with non-analyticity in the
direction (cartesian coordinates) 1.00000 0.00000 0.00000
Phonon frequencies in cm-1 :
- -1.556164E+00 6.038298E-01 1.787715E+00 2.770532E+01 4.101165E+01
- 4.388568E+01 5.063278E+01 5.872738E+01 5.890038E+01 6.873655E+01
.................................................................

Phonon at Gamma, with non-analyticity in the
direction (cartesian coordinates) 0.00000 1.00000 0.00000
Phonon frequencies in cm-1 :
- -1.556207E+00 6.025291E-01 1.787760E+00 1.230568E+01 2.770532E+01
- 4.101165E+01 4.534436E+01 5.063278E+01 5.872738E+01 6.568088E+01
.................................................................

Phonon at Gamma, with non-analyticity in the
direction (cartesian coordinates) 0.00000 0.00000 1.00000
Phonon frequencies in cm-1 :
- -1.555434E+00 6.030492E-01 1.787715E+00 1.646521E+01 2.770532E+01
- 4.101165E+01 4.388568E+01 5.063278E+01 5.872738E+01 5.890038E+01
.................................................................

The Problem is that:
calculation with non-analyticity in the direction (cartesian coordinates) 1.00000 0.00000 0.00000
is quite differing from direction 0.00000 1.00000 0.00000 and 0.00000 0.00000 1.00000 (see the 4th and 5 th mode). From experiments the 4th mode is more close to 2.770532E+01 cm-1 than to 1.230568E+01 cm-1 or 1.646521E+01 cm-1. So my questions: Can such difference exist at all? If yes: Which of these calculations (2.770532E+01 cm-1, 1.230568E+01 cm-1 or 1.646521E+01 cm-1) I have to adopt as the correct calculation in gamma point?


Thank you very much for your help!

Dear vsabadosh,

taken that your calculations are well converged: generally, non-analyticity correction depends on the direction from which one approaches the Gamma point. Do these experiments exploit this fact? I.e., do You have full phonon dispersions from experiments?

Igor Lukacevic