The ABINIT Users Mailing List ( CLOSED )

[hyello25@gmail.com]

After the calculation of phonon band structure of MgB2, I got minus phonon 
frequences at Gamma:
  Phonon at Gamma, with non-analyticity in the
  direction (cartesian coordinates)  1.00000  0.00000  0.00000
 Phonon energies in Hartree : 
  -4.397733E-04 -1.705330E-04  3.009955E-06  1.553580E-03  1.579078E-03
   1.935614E-03  2.270211E-03  2.329649E-03  3.297240E-03 
 Phonon frequencies in cm-1    : 
- -9.651908E+01 -3.742767E+01  6.606088E-01  3.409713E+02  3.465675E+02
-  4.248181E+02  4.982537E+02  5.112988E+02  7.236605E+02
     
  The convergence work has been done before to avoid unstable structure, and 
some variables of input file:

but another similar work with less stringent condition gave positive phonon 
frequence and quit good band structure. Different variables of the two files 
and part of the input file are showed below:
----minus job :
 tolvrs1   1.0d-22 
 tsmear 0.008
----positive job:
 tolvrs1   1.0d-21
 tsmear 0.03

 Can anyone give me some suggestion? Thanks in advance!
#Set 1 : ground state self-consistency
 
  getwfk1   0            # Cancel default
  kptopt1   1            # Automatic generation of k points, taking
                         # into account the symmetry
    nqpt1   0            # Cancel default
  tolvrs1   1.0d-22      # SCF stopping criterion (modify default)
  rfphon1   0            # Cancel default
 
#Q vectors for all datasets
  nqpt1 1
  qpt2 0 0 0
  qpt3 0 0 0
....(other qpts with 6x6x6 qpoints)

#Set 2 : Response function calculation of d/dk wave function
 
    iscf2   -3         # Need this non-self-consistent option for d/dk
  kptopt2   2          # Modify default to use time-reversal symmetry
  rfphon2   0          # Cancel default
  rfelfd2   2          # Calculate d/dk wave function only
  tolvrs2   0.0        # Cancel default for d/dk
  tolwfr2   1.0d-20    # Use wave function residual criterion instead
#Set 3 : Response function calculation of Q=0 phonons and electric field pert.
 
  getddk3   2          # d/dk wave functions from last dataset
  kptopt3   2          # Modify default to use time-reversal symmetry
  rfelfd3   3          # Electric-field perturbation response only
#Sets 4-10 : Finite-wave-vector phonon calculations (defaults for all 
datasets)
 
   getwfk   1          # Use GS wave functions from dataset1
   kptopt   3          # Need full k-point set for finite-Q response
   rfphon   1          # Do phonon response
  rfatpol   1 3        # Treat displacements of all atoms
    rfdir   1 1 1      # Do all directions (symmetry will be used)
   tolvrs   5.0d-11     # This default is active for sets 3-10
#######################################################################
#Common input variables
 
occopt 4
tsmear 0.008

....(cell and atom variabls)

#Definition of the planewave basis set
 
     ecut   55.0           # Maximal kinetic energy cut-off, in Hartree
     ngkpt  6  6  6
     iscf   3
  nshiftk   1             # Use one copy of grid only (default)
  shiftk   0.0 0.0 0.5