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

[Paolo Giannozzi]

On Wed, Apr 27, 2016 at 11:04 AM, Yuma Nakamura <
zcym15 at mails.tsinghua.edu.cn> wrote:
>
>
>
> With the reported set of thresholds(etot_conv_thr, forc_conv_thr, conv_thr
> and tr2_ph), 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)
>

-18 cm^-1 without Acoustic Sum Rule? it seems to me in an acceptable range
of values, not that big.

You may want to verify if your results change appreciably if you choose a
wave-vector grid that is commensurate with the k-point grid (for instance:
20 20 1 for k-points, 5 5 1 for phonon wavevectors). In some metallic
systems, characterized by the presence of semicore states, it was observed
that calculations with "commensurate" grids yield much better results than
with "uncommensurate" ones, for unclear reasons

Paolo

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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>



-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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