[QE-users] Role of alpha_mix flag in electron-phonon calculations

[Prarena Jamwal]

Dear users and developers,

I am using QE v7.0 and  trying to investigate the superconductivity of a 2D
material. I have used the PBE pseudopotential from PSlibrary and took Kmesh
36x36x1.
After vc-relax, the resulted forces and stress are:








*Total force =     0.000014     Total SCF correction =     0.000001
 Computing stress (Cartesian axis) and pressure     negative rho (up,
down):  1.001E-01 0.000E+00          total   stress  (Ry/bohr**3)
        (kbar)     P=       -0.10*

Are these values acceptable? Do I need to reduce these values?

Here a typical ph.in file I use:

















*title_line&inputph  prefix   = 'W2F2',  fildyn   = 'W2F2.dyn',  amass(1) =
 18.9984032,  amass(2) =  183.84,  outdir   = './'  ldisp    = .true.,
trans    = .true.,  fildvscf = 'dvscf', search_sym=.FALSE.,  alpha_mix(1) =
0.2,  nq1=3,  nq2=3,  nq3=1,  tr2_ph   =  1.0d-12/*

When I ran ph.in by adding "*alpha_mix flag"*, I am getting* no imaginary
frequencies*, but the lambda.out file shows NaN




















*lambda = 0.835778 (        NaN )  <log w>=      NaN K  N(Ef)= 11.024689 at
degauss= 0.005     lambda = 0.598189 (        NaN )  <log w>=      NaN K
 N(Ef)= 10.196114 at degauss= 0.010     lambda = 0.526611 (        NaN )
 <log w>=      NaN K  N(Ef)=  9.481577 at degauss= 0.015     lambda =
0.485222 (        NaN )  <log w>=      NaN K  N(Ef)=  8.817546 at degauss=
0.020     lambda = 0.465900 (        NaN )  <log w>=      NaN K  N(Ef)=
 8.336884 at degauss= 0.025     lambda = 0.466822 (        NaN )  <log w>=
     NaN K  N(Ef)=  8.111690 at degauss= 0.030     lambda = 0.481500 (
   NaN )  <log w>=      NaN K  N(Ef)=  8.111006 at degauss= 0.035
 lambda = 0.504944 (        NaN )  <log w>=      NaN K  N(Ef)=  8.267627 at
degauss= 0.040     lambda = 0.532578 (        NaN )  <log w>=      NaN K
 N(Ef)=  8.520600 at degauss= 0.045     lambda = 0.562378 (        NaN )
 <log w>=      NaN K  N(Ef)=  8.823055 at degauss= 0.050lambda
 omega_log          T_c   0.83578           NaN                NaN
 0.59819           NaN                NaN   0.52661           NaN
     NaN   0.48522           NaN                NaN   0.46590           NaN
               NaN   0.46682           NaN                NaN   0.48150
      NaN                NaN   0.50494           NaN                NaN
 0.53258           NaN                NaN*
  * 0.56238           NaN                NaN*

And when I ran ph.in *without alpha_mix flag*, there were imaginary
frequencies in the phonon dispersion and I am getting values in lambda.out
file










*lambda = 0.760811 (   0.760801 )  <log w>=  197.892 K  N(Ef)= 11.024689 at
degauss= 0.005     lambda = 0.573322 (   0.573317 )  <log w>=  188.429 K
 N(Ef)= 10.196114 at degauss= 0.010     lambda = 0.520900 (   0.520896 )
 <log w>=  184.554 K  N(Ef)=  9.481577 at degauss= 0.015     lambda =
0.488111 (   0.488108 )  <log w>=  183.224 K  N(Ef)=  8.817546 at degauss=
0.020     lambda = 0.474378 (   0.474375 )  <log w>=  182.309 K  N(Ef)=
 8.336884 at degauss= 0.025     lambda = 0.481178 (   0.481175 )  <log w>=
 181.389 K  N(Ef)=  8.111690 at degauss= 0.030     lambda = 0.502378 (
0.502375 )  <log w>=  180.618 K  N(Ef)=  8.111006 at degauss= 0.035
 lambda = 0.533389 (   0.533386 )  <log w>=  180.243 K  N(Ef)=  8.267627 at
degauss= 0.040     lambda = 0.569867 (   0.569863 )  <log w>=  180.075 K
 N(Ef)=  8.520600 at degauss= 0.045     lambda = 0.608489 (   0.608486 )
 <log w>=  180.009 K  N(Ef)=  8.823055 at degauss= 0.050*










*lambda        omega_log          T_c   0.76081       197.892
 7.177   0.57332       188.429              2.921   0.52090       184.554
           1.950   0.48811       183.224              1.439   0.47438
182.309              1.243   0.48118       181.389              1.329
 0.50238       180.618              1.624   0.53339       180.243
   2.106   0.56987       180.075              2.730   0.60849       180.009
             3.443*


Could anyone tell me what was going wrong here.
Kindly help me to fix this.

Thank you.
Regards
Prarena Jamwal
Research Scholar
IIT Ropar, INDIA.

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