The ABINIT Users Mailing List ( CLOSED )

[Jorge Iniguez <jiniguez@nist.gov>]

Dear abinitioners,

This email is a (long-due) follow up to an early-August discussion on
phonon calculations for MgB2. (Sorry for the delay. Summer is summer!)
The problem then was that I was not getting the right degeneracies for the
phonons (even though abinit was recognizing the right symmetry). As Lu
Fu-Fa pointed out, the solution was to specify kptopt=2 in the Gamma
phonon calculation.  I do not yet understand why kptopt=2 solves the
problem (which is probably related with the construction of the dynamical
matrix from an irreducible set of distortions), but OK, it works now.

HOWEVER, once kptopt is set to 2 and the degeneracies recovered, there is
the remaining problem of the acoustic-mode frequencies.  One obtains the
following Gamma phonons for MgB2:

  Phonon wavevector (reduced coordinates) :  0.00000  0.00000  0.00000
 Phonon energies in Hartree :
   8.717196E-04  8.717208E-04  1.195609E-03  2.028753E-03  2.028757E-03
   3.068304E-03  3.068306E-03  3.335226E-03  4.394181E-03

where the system is treated as a metal, cutoff=1000eV and k-point grid is
16x16x12.  The acoustic frequencies are terribly large! (around 200 cm-1)

If I increase the cutoff to 1500 eV, I get:

  Phonon wavevector (reduced coordinates) :  0.00000  0.00000  0.00000
 Phonon energies in Hartree :
   8.718879E-04  8.719018E-04  1.195568E-03  2.029070E-03  2.029101E-03
   3.070568E-03  3.070573E-03  3.335321E-03  4.394402E-03

which is the same result (cutoff=2000eV does not change anything).  If I
use an even denser grid of 20x20x16 k points, I get:

  Phonon wavevector (reduced coordinates) :  0.00000  0.00000  0.00000
 Phonon energies in Hartree :
   8.723311E-04  8.723325E-04  1.195953E-03  2.031243E-03  2.031243E-03
   3.045633E-03  3.045633E-03  3.340805E-03  4.400362E-03

So, the calculation was very well converged with the initial conditions.

Now, if I treat the system as an *insulator*, for 1000 eV and 16x16x12, I
get:

  Phonon wavevector (reduced coordinates) :  0.00000 0.00000 0.00000
 Phonon energies in Hartree :
  -1.347742E-05 -1.098398E-05  4.037062E-06  1.539327E-03  1.539351E-03
   1.760802E-03  3.431914E-03  4.124680E-03  4.124681E-03

with reasonably small acoustic frequencies!!!  So, it seems there is
something funny about the acoustic phonons in the metallic case. I guess
this is not an input-file problem...  (a typical input file goes
attached).  Any suggestions?

Cheers,
Jorge  acell   5.7690981162E+00  5.7690981162E+00  6.6066862442E+00
  angdeg  90 90 120
  xred
  0 0 0
  1/3 2/3 1/2
  2/3 1/3 1/2

natom 3
ntype 2
znucl  12 5
type  1 2*2

# xc, cutoff and k points
ixc 1
ecut 1000 eV
kptopt 1
ngkpt  16 16 12
nshiftk 1
shiftk 2*0.0 0.5

occopt 3
tsmear 0.1 eV

nstep 100

#####

ndtset 2
jdtset 1 3

### DS 1 Getting very well converged wave functions (needed for RF calcs)

tolvrs1   1.0d-18
iscf1    5
prtden1 1
prtwf1 1

### DS 2 Response function calculation d/dk (for dynamical matrix at Gamma)

rfelfd2        2              # Activate calc of d/dk perturbation
rfdir2         1 1 1           # Consider all three directions

nqpt2      1
qpt2       0.0 0.0 0.0   # Calc at the Gamma point

getwfk2     1      # WFs from DS 2

iscf2       -3      # d/dk treated in a non-self-consistent way

tolwfr2      1.0d-22


### DS3 Response function calculation: E-field perturbation and phonons

kptopt3 2
rfphon3      1         # Atomic displacement perturbation calc activated
rfatpol3     1 3       # All atoms will be displaced
#rfelfd3      3         # E-field perturbation calc activated
rfdir3       1 1 1     # All three directions considered

nqpt3      1
qpt3       0.0 0.0 0.0   # Gamma point calc

getwfk3      1      # WFs from DS 1
#getddk3      2

tolvrs3      1.0d-7
iscf3        5      # Self-consistent calc using algthm 5