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.cant...@spin.cnr.it <giovanni.cant...@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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