[Pw_forum] Negative frequencies in proximity to the Gamma point problem

[Mohammad Alidoosti]

Dear PWSCF users,

I am doing phonon frequencies calculations for Graphene-oxide. In proximity
to the Gamma point, I get small negative frequencies. As far as I know this
is a known problem, I have tried to increase "forc_conv_thr" and "tr2_ph"
to 1.0d-6 and 1.0d-16, respectively, but it doesn't work. Is there any
suggestion for solving this problem?
Hear is the scf input file :
*********************************************************************
 &control
  title = GO
   calculation = 'scf'
    restart_mode = 'from_scratch'
    prefix = 'GO-ph'
    pseudo_dir = '/home/pseudo'
    outdir = './tmp/'
                      nstep  = 200 ,
                   tstress = .true. ,
                     tprnfor = .true. ,
                    verbosity='high',
                      forc_conv_thr= 1.0d-6
 /
 &system
    ibrav = 0
    nat = 12
    ntyp = 3
    ecutwfc = 80
    ecutrho = 400
    occupations = 'smearing'
    smearing='methfessel-paxton',
    degauss = 0.001
/
 &electrons
conv_thr= 1.0d-8
mixing_beta = 0.7
 electron_maxstep = 200,
 /
 &IONS
/
&CELL
cell_dofree = '2Dxy',
/
ATOMIC_SPECIES
  C   12.0107    C.pbe-n-kjpaw_psl.0.1.UPF
  O   15.999  O.pbe-n-kjpaw_psl.0.1.UPF
  H   1.008   H.pbe-kjpaw_psl.0.1.UPF
CELL_PARAMETERS (angstrom)
   4.990235060   0.054948389   0.000000000
  -2.541880421   4.219875071   0.000000000
   0.000000000   0.000000000  25.000000000

ATOMIC_POSITIONS (angstrom)
O       -0.021826634   0.250928937   1.721705471
H        0.809321037   0.681392784   1.997855886
O       -0.031523808   1.135550155  -1.892027240
H        0.808877574   0.723408478  -2.168118129
C        0.047350145   0.017290716   0.288486347
C        0.032454255   1.370630042  -0.458840404
C       -1.237500438   2.119007372  -0.142173618
C       -1.224236423   3.485316018  -0.033121650
C        1.242549808   3.515955831  -0.028144886
C        1.285890831   2.150271662  -0.137183942
C        2.528922921   1.430926894  -0.090219298
C        2.544546281   0.011979550  -0.080086702
K_POINTS automatic
15 15 1 0  0  0
*******************************************************************

Thanks in advance.

Mohammad Alidoosti
-- 
Institute for Research in Fundamental Sciences (IPM)
School of Nano-Science
Shahid Farbin Alley
Shahid Lavasani st
P.Code: 19538-33511
Tehran, Iran
Tel: +98 21 2310 -3070
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