[Pw_forum] Negative phonon frequency for 2D material of Sn (stanene)

[Yuma Nakamura]

Dear All 






I started Quantum Espresso half years ago so I am relatively beginner user.   
Now I am trying to reproduce the phonon dispersion of  Stanene(graphene-like tin mono-layer) according to the article[S. Jomehpour Zaveh et al.,Superlattices and Microstructures 91 (2016) 383].






With the reported set of thresholds(etot_conv_thr, forc_conv_thr, conv_thr and tr2_ph),the negative frequency appears at Gamma point.
It seems that this negative frequency is not small enough to be modified by acoustic sum rule. (please check the output below)




I understand this is because its structure is not stable.
In the final relaxed state, the stress is -0.02 kbar in xx(yy) component and -0.24 kbar in zz component.        


Is it optimization problem or do I miss some important control tags?




I used in Quantum Espresso 5.2.1


Here is my trial set of thresholds and result.




————————————————optimization & scf————————————
calculation mode = vc-relax 
ion_dynamics     = bfgs                 
ecutwfc          = 65     *
etot_conv_thr    =1.0D-12 *         
forc_conv_thr    =1.0D-6  *          
conv_thr         =1.0D-14 
k-point =21x21x1 *
————————————————ph run———————————————————————— 
tr2_ph           =1.0D-16
q-point          =5x5x1   *
——————————————————————————————————————————————


* theses values are referred from the literature, the other values are not mentioned and determined by myself.






     Diagonalizing the dynamical matrix


     q = (    0.000000000   0.000000000   0.000000000 )


 **************************************************************************
     freq (    1) =      -0.551779 [THz] =     -18.405359 [cm-1]
     freq (    2) =      -0.551779 [THz] =     -18.405359 [cm-1]
     freq (    3) =       0.352400 [THz] =      11.754783 [cm-1]
     freq (    4) =       3.460541 [THz] =     115.431227 [cm-1]
     freq (    5) =       5.407571 [THz] =     180.377167 [cm-1]
     freq (    6) =       5.407571 [THz] =     180.377167 [cm-1]
 **************************************************************************
     Mode symmetry, C_3v (3m)   point group:


     freq (  1 -  2) =        -18.4  [cm-1]   --> E    L_3        I+R
     freq (  3 -  3) =         11.8  [cm-1]   --> A_1  L_1        I+R
     freq (  4 -  4) =        115.4  [cm-1]   --> A_1  L_1        I+R
     freq (  5 -  6) =        180.4  [cm-1]   --> E    L_3        I+R




In those setups, I got converged results(even phonon frequency is negative).




The complete input files are following. The output files are attached.






—————————————————————pw.x———————————————————————————— 
 &control
    calculation  = 'vc-relax'      
    restart_mode = 'from_scratch'  
    outdir       = './'
    prefix       = 'stanene'       
    tstress      = .true.          
    tprnfor      = .true.          
    pseudo_dir   = './'
    verbosity    = 'high'
    etot_conv_thr = 1.0E-12
    forc_conv_thr = 1.0D-6


 /
 &system
    ibrav     = 0
    celldm(1) = 8.83069022
    nat       = 2
    ntyp      = 1
    ecutwfc   = 65
    ecutrho   = 600
    degauss   = 0.02
    smearing  ='mp'
 occupations  ='smearing',






/
 &electrons
    diagonalization = 'david'
    mixing_mode     = 'plain'
    mixing_beta     = 0.7
    conv_thr        = 1.0d-14
 /
 &ions
    ion_dynamics = 'bfgs'
 /
 &cell
    cell_dofree = '2Dxy'
 /
ATOMIC_SPECIES
Sn  118.710 Sn.pbe-dn-rrkjus_psl.0.2.UPF
CELL_PARAMETERS (alat=  8.83069022)
   1.002153232   0.000000000   0.000000000
  -0.501076616   0.867890158   0.000000000
   0.000000000   0.000000000   4.061049000
ATOMIC_POSITIONS {crystal}
Sn       0.666666666   0.333333333  -0.000019757
Sn       0.333333333   0.666666667   0.044809919
K_POINTS automatic
36 36 1 0 0 0
—————————————————————ph.x————————————————————————————
phonons of Sn
 &inputph
  tr2_ph=1.0d-16,
  prefix='stanene',
  ldisp=.true.,
  nq1=5, nq2=5, nq3=1
  start_q=1   
  last_q=1    
  amass(1)=118.710,
  outdir=“.//1”,
  fildyn='stanene.dyn',
 /








Best regards,
+++++++++++++++++
Yuma Nakamura
Double Master Degree candidate 
Dept of Chemistry in Tsinghua University, China
Dept of Physics   in Tohoku   University, Japan








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