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

[mkondrin at hppi.troitsk.ru]

Dear Giuseppe, I believe that your bilayer MoS2 structure is not fully 
relaxed. What is the interaction between VdW bound layers? How it is 
accounted for in your calculations? So, it can be possible that negatve 
frequencies are just very small positive frequencies which are not 
precisely calculated.

I am nor sure that VdW interaction can be accounted for in phonon 
calculations.

Sincerely yours, 
M. V. Kondrin 

On Aug 3 2023, Giuseppe Scala wrote:

>Hi ANAND,
>
>Would you mind if I ask you to tell me where's the problem in the
>structure? Maybe that c parameter is too high or low?
>Il gio 3 ago 2023, 12:58 ANAND JHA <anandkj21 at iitk.ac.in> ha scritto:
>
>> There is some issue in your bilayer
>> Try to make a new bilayer with CRYSTAL SOFTWARE
>>
>> On Thu, 3 Aug 2023 at 03:55, Giuseppe Scala <g.scala2 at campus.unimib.it>
>> wrote:
>>
>>> 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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>>
>> _______________________________________________
>> The Quantum ESPRESSO community stands by the Ukrainian
>> people and expresses its concerns about the devastating
>> effects that the Russian military offensive has on their
>> country and on the free and peaceful scientific, cultural,
>> and economic cooperation amongst peoples
>> _______________________________________________
>> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
>> users mailing list users at lists.quantum-espresso.org
>> https://lists.quantum-espresso.org/mailman/listinfo/users
>