[QE-users] Negative frequencies for phonon dispersion in Bilayer MoS2

[Giuseppe Scala]

I get negative frequencies at gamma points for Bilayer MoS2. I relaxed my
structure and checked convergence with respect to energy cutoff. That's my
input file for the scf calculation:

##############

&CONTROL
calculation ='scf'
prefix = 'MoS2_BI'
pseudo_dir = '/home/Giuseppe'
outdir = './'
tstress = .true.
tprnfor = .true.
forc_conv_thr = 1.0d-3
nstep = 200
/

     &SYSTEM
ibrav = 4
a = 3.1429635960300000000
c = 20.006663000000000000
nat = 6
ntyp = 2
ecutwfc = 60
ecutrho = 240
occupations = 'smearing'
degauss = 0.01
/

    &ELECTRONS
mixing_beta = 0.7
conv_thr = 1.0d-8
diagonalization = 'david'
electron_maxstep=200
/

    &IONS
trust_radius_max = 0.2
/
    &CELL
cell_dynamics = "bfgs"
cell_dofree = '2Dxy'
/

ATOMIC_SPECIES
Mo 95.9400 Mo.pbesol-spn-kjpaw_psl.1.0.0.UPF
S 32.0660 S.pbesol-n-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS {angstrom}
Mo 1.5714815237 0.9072955423 6.6890512735
Mo 0.0000000000 1.8145901001 13.377562735
 S 1.5714815237 0.9072955423 11.825385701
 S 1.5714815237 0.9072955423 14.928508325
 S 0.0000000000 1.8145901001 8.2412092563
 S 0.0000000000 1.8145901001 5.1381015776

K_POINTS {automatic}
4 4 4 0 0 0

#########


And here my input file for ph.x


#######

&inputph
prefix = 'MoS2_BI'
fildvscf = 'dvscf'
ldisp = .true.
fildyn = 'MoS2_BI.dyn'
alpha_mix(1) = 0.3
nq1 = 2,
nq2 = 2,
nq3 = 2,
tr2_ph = 1.0d-12
/
########

Is my bilayer wrong? I've come the exact same calculations for Bulk and
Monolayer MoS2 and everything went smoothly
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