[Pw_forum] negative phonon frequencies again :: graphene

[Martin Gmitra]

Dear all,

Let me open question of negative (imaginary) phonon frequencies one
more time to the forum.
I would like to calculate phonon dispersions for graphene. I can not
get rid of negative frequencies
for ZA mode in vicinity of Gamma point. Please have a look at attached
figure, where different USPP
with LDA and GGA have been used for different energy cut-offs. One
gets impression that increasing
q-mesh for interpolation removes the problem but it is not a case. I
have found a post (no further replies)
http://qe-forge.org/pipermail/q-e-developers/2010-June/000030.html
where the phonon dispersion does not suffer from the negative
frequencies at all. I would appreciate
if Huang Shouting provide more details of his inputs. I have to say
that I have tried many variations
of input parameters like (using C.pbe-n-rrkjus_psl.0.1.UPF if not
specified else):
-- experimental lattice constant and relaxed within used PP
-- variation of c/a, increasing vacuum up to c/a=13
-- with and without inversion symmetry (shifting atomic z positions out of zero)
-- k-mesh for SCF calculation containing K_point=(0.3333333 0.57735026
0) (yield very wrong phonons)
and without the K_point
-- k-mesh with and without shift (turning on/off Gamma point)
-- varying ecutwfc and ecutrho parameters for single mode calculation
q=(0.0  0.01  0.0) to inspect convergence,
see middle figure, for different PPs and cut-offs/
-- decreasing tr2_ph threshold for single mode calculation q=(0.0
0.01  0.0), bottom figure, does not make
the frequencies positive for selected negative frequency (ecutwfc=57,
ecutrho=340), see arrow at middle figure.

None of this variation brings me to success. Could you please comment
or give some advices?

Best regards,
Martin Gmitra

Universität Regensburg
Institut I - Theoretische Physik
Universitätsstraße 31
D-93040 Regensburg
Deutschland/Germany


Below are examples of input files used.

----------------------------------------------------------------------------------------------

 &CONTROL
                 calculation = 'scf' ,
                restart_mode = 'from_scratch' ,
                      outdir = './tmp/' ,
                  pseudo_dir = './espresso/pseudo/' ,
                      prefix = 'grp' ,
 /
 &SYSTEM
                       ibrav = 4,
                   celldm(1) = 4.65857947376452 ,
                   celldm(3) = 6.09876841964 ,
                         nat = 2,
                        ntyp = 1,
                     ecutwfc = 37 ,
                     ecutrho = 320 ,
                        nbnd = 8 ,
 /
 &ELECTRONS
                    conv_thr = 1.0d-10 ,
                 mixing_mode = 'plain' ,
                 mixing_beta = 0.7 ,
             diagonalization = 'david' ,
 /
ATOMIC_SPECIES
    C   12.01078  C.pbe-n-rrkjus_psl.0.1.UPF
ATOMIC_POSITIONS crystal
    C      0.333333333    0.666666666    0.500000000   1  1  0
    C      0.666666666    0.333333333    0.500000000   1  1  0
K_POINTS automatic
 13 13 1   0 0 0

----------------------------------------------------------------------------------------------

 &INPUTPH
                      outdir = './tmp/' ,
                      prefix = 'grp' ,
                      fildyn = 'grp_nearG.dyn' ,
                       trans = .true.,
                alpha_mix(1) = 0.7,
                    amass(1) = 12.01078,
                  iverbosity = 1 ,
                      tr2_ph = 1.0d-12 ,
                     nmix_ph = 4,
 /
0.0  0.01  0.0
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