[QE-users] Issue with the convergence of superconductivity Tc for MgB2

[saqib]

Dear QE users,

I am trying to calculate MgB2 superconducting Tc with lambda.x. As I understand it, the correct value will be the one that is converged with respect to sigma value (within plateau). However, I get a strange results without getting any plateau and therefore can't pick the correct value. Values of sigma are default (from 0.005 to 0.1). The output of lamda.x is given below:

 lambda        omega_log          T_c
   0.69701       640.588             22.047
   0.74121       644.652             25.642
   0.72855       644.807             24.658
   0.71480       644.406             23.568
   0.70317       644.049             22.646
   0.68885       644.102             21.531
   0.67095       644.454             20.150
   0.65095       645.095             18.622
   0.63044       645.881             17.069
   0.61045       646.888             15.579

I will really appreciate if QE users/experts can help me resolve this problem or highlight the correct procedure of picking Tc value.
Both scf and phonon calculations are nicely converged so I don't think convergence is a problem. Input for scf and phonon calculations are given below:

scf
 &control
    calculation='scf',
    prefix='MgB2',
    pseudo_dir = '~/pseudo/',
    outdir='./save/',
    tprnfor = .true.,
    tstress = .true.,
    etot_conv_thr = 1.0d-5
    forc_conv_thr = 1.0d-4
 /
 &system
    ibrav = 4,
    celldm(1) = 5.8260252227888,
    celldm(3) = 1.1420694129095,
    nat=  3,
    ntyp = 2,
    ecutwfc = 100
    smearing = 'mp'
    occupations = 'smearing'
    degauss = 0.02
la2F = .true.,
 /
 &electrons
    diagonalization = 'david'
    mixing_mode = 'plain'
    mixing_beta = 0.7
    conv_thr =  1.0d-12
 /
ATOMIC_SPECIES
 Mg  24.305  Mg.pz-n-vbc.UPF
 B   10.811  B.pz-vbc.UPF
ATOMIC_POSITIONS crystal
Mg       0.000000000   0.000000000   0.000000000
B        0.333333333   0.666666667   0.500000000
B        0.666666667   0.333333333   0.500000000
K_POINTS AUTOMATIC
16 16 16 0 0 0

phonon

e-ph MgB2
&inputph
  prefix   = 'MgB2',
  fildyn   = 'MgB2.dyn',
  amass(1) = 24.305,
  amass(2) = 10.811,
  outdir   = './save/'
  ldisp    = .true.,
  trans    = .true.,
  fildvscf = 'dvscf',
  nq1=8,   nq2=8,   nq3=8,
  tr2_ph   =  1.0d-14
electron_phonon='interpolated',
  el_ph_sigma=0.005,
  el_ph_nsigma=10,
 /


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