[QE-users] Electron Phonon Calculation: tolerance limit for P

[Bidault, Xavier]

Hello Ramesh,

Norm-conserving (NC) pseudopotentials (PP) require higher energy cutoffs. You need NC PP for electron-phonon calculations. NC PP from PseudoDojo website are very good, at least for what I've tested on organic materials. Then you don't need ecutrho for NC PP, which are made to work with default ecutrho = 4 x ecutwfc.

So, you should try first to increase your cutoff, up to 200 Ry should do the job to have the asymptotic behavior with PseudoDojo NC PP (unfortunately, other NC PP may require much higher cutoffs to get the asymptotic behavior). Then you can choose the cutoff where the asymptotic error is just below 1 meV/atom.

After you do a vc-relax simulation, always check the verification step at the end of the output. If the pressure tensor is different from the pressure tensor of the last optimization step, then it means your cutoff is not large enough, or your system has converging issues (initial geometry, k-points...).

I think your forc_conv_thr of 1e-4 is good. You can tighten a little bit etot_conv_thr to 1e-5 and press_conv_thr to 0.1 kbar.

Regards,
Xavier
________________________________
From: users <users-bounces at lists.quantum-espresso.org> on behalf of Ramesh Kumar Kamadurai via users <users at lists.quantum-espresso.org>
Sent: Thursday, October 27, 2022 3:23 AM
To: users at lists.quantum-espresso.org <users at lists.quantum-espresso.org>
Subject: [QE-users] Electron Phonon Calculation: tolerance limit for P

Dear users
I am trying to calculate el-Ph coupling constant and other properties for a superconductor. The input file is attached. I am a bit stuck with VC-Relax calculation as the system is not able to stabilize at 0.0 kbar. I followed all the suggestions given in forum
1. Increased Cut-off to maximum
2. Tried USPP,NC, Mixed and currently running with PBE-Sol
3. Sequentially done 2 VC Relax calculation but i am not able to sort out the issue.
&CONTROL
    calculation   = "vc-relax"
    etot_conv_thr =  4.00000e-05
    forc_conv_thr =  1.00000e-04
    max_seconds   =  1.72800e+05
    nstep         = 100
    outdir        = "/home/sjacobs/tmp/"
    prefix        = "PDZ-VCRELAX"
    pseudo_dir    = "C:\Users\RAMESH\.burai\.pseudopot"
    tprnfor       = .TRUE.
    tstress       = .TRUE.
    verbosity     = "high"
/

&SYSTEM
    a           =  6.36512e+00
    degauss     =  7.34986e-03
    ecutrho     =  6.40000e+02
    ecutwfc     =  8.00000e+01
    ibrav       = 2
    nat         = 4
    nosym       = .FALSE.
    nspin       = 1
    ntyp        = 3
    occupations = "smearing"
    smearing    = "marzari-vanderbilt"
/

&ELECTRONS
    conv_thr         =  8.00000e-10
    electron_maxstep = 80
    mixing_beta      =  4.00000e-01
    startingpot      = "atomic"
    startingwfc      = "atomic+random"
/

&IONS
    ion_dynamics = "bfgs"
/

&CELL
    cell_dofree    = "all"
    cell_dynamics  = "bfgs"
    press          =  0.00000e+00
    press_conv_thr =  5.00000e-01
/

K_POINTS {automatic}
12 12 12  0 0 0

ATOMIC_SPECIES
Ga     69.72300  Ga.pbesol-dnl-rrkjus_psl.1.0.0.UPF
Pd    106.42000  Pd_ONCV_PBEsol-1.0.upf
Zr     91.22400  zr_pbesol_v1.uspp.F.UPF

ATOMIC_POSITIONS {crystal}
Zr      0.000000   0.000000   0.000000
Ga      0.493162   0.493162   0.493162
Pd      0.739743   0.739743   0.739743
Pd      0.246581   0.246581   0.246581

The output stress is given below
entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=    2.67
     entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.41
     entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.82
     entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.56
     entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.13
     entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.08
     entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.23
     entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.35
     entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.50
     entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.63
     entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.77
     entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.83
My question is
What is the optimal pressure value to avoid negative frequencies for electron phonon calculation ?
Is there any mistake in my input file ?
Thanks in advance
Regards
Ramesh
Asst. Professor
GITAM University
Vizag, India

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