[QE-users] No or band convergence for WS2 bilayer

[Giovanni Cantele]

Dear all,

I'm facing a bad convergence issue for a very simple system consisting of a
WS2 bilayer.

While with NO vdW interaction turned on everything works fine, as I use the
input_dft variable the optimization task fails. In practice, the pw.x tries
to optimize the bilayer for some steps, when the Total force decreases,
then the Total force starts increasing, until at some point (=geometry),
the electron minimization does not converge within electron_maxstep = 300
steps.

Here, a typical input file I use:

&CONTROL
    calculation   = 'relax'
    title         = 'WS1-2L'
    restart_mode  = 'from_scratch'
    pseudo_dir   = '/m100_work/IscrB_TWITTER/pseudo/'
    outdir        = './tmp/'
    prefix        = 'WS2-2L'
    verbosity     = 'high'
    tprnfor       = .true.
    tstress       = .true.
    !etot_conv_thr = 1.D-5
    forc_conv_thr = 1.D-6
/
&SYSTEM
    ibrav             = 4
    a                 = __A
    c                 = 24.450
    nat               = 6
    ntyp              = 2
    ecutwfc           = 50
    ecutrho           = 500
    occupations       = 'smearing'
    smearing          = 'mv'
    degauss           = 0.00735
    input_dft         = 'vdw-df2-b86r'
/
&ELECTRONS
    conv_thr         = 1.d-8
    electron_maxstep = 300
    mixing_beta      = 0.15
/
&IONS
/
ATOMIC_SPECIES
        W    183.84       W.pbe-spn-kjpaw_psl.1.0.0.UPF
        S     32.065      S.pbe-n-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS { crystal }
S    0.666666667   0.333333333   0.080882686
W    0.333333333   0.666666667   0.145220672
S    0.666666667   0.333333333   0.209558657
S    0.333333333   0.666666667   0.371324029
W    0.666666667   0.333333333   0.435662015
S    0.333333333   0.666666667   0.500000000
K_POINTS { automatic }
12  12  1    0  0  0


I tried to decrease the beta (down to 0.01), to increase degauss (even
though the system is semiconducting and this should not affect the
convergence, since the energy gap is always much larger than degauss), to
increase the k-point grid, to increase or decrease the vacuum, to increase
ecutrho.

I suspect some stupid error I'm doing and that I cannot see, since most of
the above listed recipies has worked for much more difficult systems that a
simple WS2 bilayer, unless WS2 requires some special care that I ignore, or
unless there is some issue with the W and/or S pseudopotential.
I also tried to run the run with no vdw and then that with vdw as a restart
of the previous one.

WS2 bilayer seems for example to have been successfully studied with
several (and even more recent) vdW functionals for example in this paper:
https://pubs.acs.org/doi/pdf/10.1021/acs.jctc.0c00471

I would appreciate any hint/help.

Thank you.
Giovanni

-- 

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it <giovanni.cantele at spin.cnr.it>
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: https://sites.google.com/view/giovanni-cantele/home
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