Dear Stefano, Thank you very much for your prompt response, that helps a lot! I will repeat the tests as you suggested but it's good to know that my system is behaving normally and I can then proceed with more complex calculations.
All the best, Martina On Tue, Apr 24, 2018 at 3:23 AM, Stefano de Gironcoli <[email protected]> wrote: > Dear Martina Lessio, > > first of all I would say that a convergence error of 1.d-5 Ry in a 6 > atom cell looks pretty much converged to me. I think that even your ecutrho > = 240 Ry calculation (~1.d-5 Ry per atom) looks quite good. > > coming to the way things converge: > > - total energy convergence with respect to ecutwfc is expected to be > from above because of variational principle: the higher ecutwfc is the > more planewaves are included in the wavefunction expansion, hence the lower > the energy. However in the ultrasoft pseudopotential definition the energy > is not only a function of the wavefunctions but also includes a dependence > on augmentation charges, that are localized and may contain higher Fourier > components with respect to 4*ecutwfc ( = 240 in your case). Failing to > include enough Fourier components in the augmentation charges will affect a > number of integrals but not in a variational way... integrals would simply > be inaccurate and the inaccuracy can be both from above or from below. > > - I would perform cutoff convergence test in a slightly different > order: 1) I would check convergence of total energy (and stress, and > forces) as a function of ecutwfc using the default value for > ecutho=4*ecutwfc (that is without specifying ecutrho in the input). When > this procedure converges (and it can initially converge from below due to > augmentation charge Fourier components being missing) this means that > wavefunction expansion AND augmentation-charge expansion are both > converged. 2) I would then fix ecutrho=4*converged_ecutwfc, which takes > care of augmentation charge convergence, and I would check whether I can > get away with a lower ecutwfc for the wavefunction expansion. > > - as for k-point sampling convergence, there is no variational > principle w.r.t. number of k-points: it's again a matter of convergence of > an integral. The denser the grid the better the integral but there is no > variational principle with respect to which k-point you include and which > you dont. > > hope this helps > > stefano > > On 24/04/2018 05:56, Martina Lessio wrote: > > Dear Quantum Espresso community, > > I am new to Quantum Espresso and I am trying to run some simple > simulations on MoTe2 bulk. Unfortunately I seem to be having some issues > with some preliminary convergence tests for charge density cutoff and > K-point grid and I am hoping to get some help from you on this. > Here is a graph with the results of the charge density cutoff convergence > test I performed while setting the kinetic energy cutoff equal to 60 Ry (I > performed a test to set this as well): > > I am worried about these results because I would expect the total energy > to go down rather than going up when I increase ecutrho. I also observe a > similar energy trend when I increase the k-point grid, which also seems > unusual and possibly wrong to me. > I am copying below the input I have used for these calculations and I > would greatly appreciate any help with figuring our whether I am doing > something wrong. > > Thank you so much! > > Kind Regards, > Martina Lessio > > Postdoctoral Research Scientist > Department of Chemistry > Columbia University > > &control > > calculation = 'scf' > > restart_mode='from_scratch', > > prefix='MoTe2_ecutwfc', > > pseudo_dir = '/home/mlessio/espresso-5.4.0/pseudo/', > > outdir='/home/mlessio/espresso-5.4.0/tempdir/' > > / > > &system > > ibrav= 4, A=3.530, B=3.530, C=13.882, cosAB=-0.5, cosAC=0, cosBC=0, > > nat= 6, ntyp= 2, > > ecutwfc =60.0 ecutrho=300. > > nspin =4, lspinorb =.true., noncolin=.true. > > / > > &electrons > > mixing_mode = 'plain' > > mixing_beta = 0.7 > > conv_thr = 1.0d-8 > > / > > ATOMIC_SPECIES > > Mo 95.96 Mo.rel-pbe-spn-rrkjus_psl.1.0.0.UPF > > Te 127.6 Te.rel-pbe-n-rrkjus_psl.1.0.0.UPF > > ATOMIC_POSITIONS {crystal} > > Te 0.333333334 0.666666643 0.625000034 > > Te 0.666666641 0.333333282 0.375000000 > > Te 0.666666641 0.333333282 0.125000000 > > Te 0.333333334 0.666666643 0.874999966 > > Mo 0.333333334 0.666666643 0.250000000 > > Mo 0.666666641 0.333333282 0.750000000 > > > K_POINTS {automatic} > > 8 8 2 0 0 0 > > > _______________________________________________ > users mailing > [email protected]https://lists.quantum-espresso.org/mailman/listinfo/users > > > > _______________________________________________ > users mailing list > [email protected] > https://lists.quantum-espresso.org/mailman/listinfo/users > -- Martina Lessio, Ph.D. Frontiers of Science Lecturer in Discipline Postdoctoral Research Scientist Department of Chemistry Columbia University
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