The ABINIT Users Mailing List ( CLOSED )

[PGanesh <pganesh@ciw.edu>]

Hi All,

I am trying to compute the phonon band structure. Using RF method I 
computed the second derivatives
using a gamma centered q-mesh 6 6 6 (a seperate self-consistent run for 
each qpt. using the GS wavefunctions) and a shifted k-mesh 12 12 12 
(from reading papers I think thats enough).  The calculations are at a 
high presssure of about 100GPa. Then I used "anaddb" program to compute 
the full band-structure by interpolating the IFC's in the fourier 
space.  I get an accoustic mode to soften and go to negative frequency 
at a high symmetry point (N which is along cubic [110] with reduced 
coordinates (0,0,0.5) ).  But if I check the frequencies at the q-points 
in the grid I found that they are all positive.  So I don't understand 
why after interpolation I can get a negative value when all points on 
the grid have positive frequencies? So I did a separate run at that 
exact 'q' point (N) and found that it is not negative. In fact it is 
highly positive.  Has anyone found similar problems using "anaddb" ? Any 
suggestions will be very helpful.  Is there a way to printout the 
dynamical matrix at an arbtrary q-pt while using the "anaddb" program?

Thanks in advance.

Ganesh