[Pw_forum] Irregular phonon dispersion

[Cameron Foss]

I am trying to calculate phonons in monolayer (ML) mos2 along high symmetry
paths, I am using the same input file that gave me a good electronic band
structure. I am using espresso-5.1. The scf and ph input files are pasted
below. I have also attached a .tif file of the dispersion, please notice
that near the Gamma points there is some significant irregularity.

I am curious to know why this input file worked OK for the electronic band
structure but not so well for phonons? For clarity I used an 8x8x1 MP grid
for the electronic band structure calculation and a 6x6x4 MP grid for
phonons. (I had good success using a 6x6x4 MP grid for calculating phonons
in ML mos2 before however I used the wrong lattice constants putting the
structure under strain thus the purpose of this new calculation).

I understand the vdW forces may play a role in electronic band calculations
of MLs so the interplanar distance is ~15 angstroms for both the electronic
band calculation and phonons calculation.

%%%%%%%%%%%%SCF CALC
&CONTROL
    calculation='scf',
    restart_mode='from_scratch',
    pseudo_dir='/home/cf79a/espresso-par/espresso-5.1.2/pseudo',

outdir='/home/cf79a/espresso-par/espresso-5.1.2/materials/EFRI/outdir/MoS2',
    prefix='MoS2-lda',
/
&SYSTEM
    ibrav=4, celldm(1)=5.905393714, celldm(3)=6,
    nat=3, ntyp=2, ecutwfc =140
/
&ELECTRONS
    conv_thr =  1.0d-15
    mixing_beta = 0.7
/
ATOMIC_SPECIES
 Mo  95.94    Mo.pw-mt_fhi.UPF
 S   32.065   S.pw-mt_fhi.UPF
ATOMIC_POSITIONS bohr
S        0.00002      -0.00001       0.000000000
Mo       2.952696857   1.704740325   2.938523912
S        0.00002      -0.00001       5.877047824
K_POINTS automatic
  6 6 4 1 1 1

%%%%%%%%%%%%%%%%%PHONONS
phonons of MoS2
&inputph
  recover=.true.,
  tr2_ph=1.0d-12,
  ldisp=.true.,
  nq1=6, nq2=6, nq3=4
  amass(1)=95.94,
  amass(2)=32.065,
  prefix='MoS2-lda',
  !outdir='directory where large files are written/'

outdir='/home/cf79a/espresso-par/espresso-5.1.2/materials/EFRI/outdir/MoS2',
  fildyn='mos.dyn',
 /

Best,
Cameron
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