Dear Stefano, Thanks so much for the clarification. I am very grateful for your prompt response!
All the best, Martina On Wed, Apr 25, 2018 at 5:16 PM, Stefano de Gironcoli <[email protected]> wrote: > Dear Martina Lessio, > > the 8*ecutwfc is a rule of thumb that is indeed often suggested for US > pseudopotential. > > There is nothing special in the factor 8, it simply reflects the > expectation that augmentation charges typically contain more Fourier > components than the 4*ecutwfc coming from the square of the wavefuncitons. > > As you did the converge test and found that 55/280 Ry are good in your > case it should be ok to use these values. > > best, > > stefano > > On 25/04/2018 22:11, Martina Lessio wrote: > > Dear Stefano, > > I have one more question regarding the convergence tests. I have performed > the tests in the order that you recommended and found that if I set > ecutrho=280 Ry I can get away with ecutwfc=55 Ry without negatively > impacting the convergence. So I am thinking of using these parameters for > my future calculations. However, I have read in many QE resources that if > you use ultrasoft pseudopotentials like I do, ecutrho should be at least > equal to 8*ecutwfc. So I am now wondering whether it is an issue that in my > case ecutrho is only equal to about 5*ecutwfc or given that I performed the > recommended convergence tests it is okay to have this setup even for > pseudopotentials. > > I hope my question is clear. > Thanks so much! > > All the best, > Martina > > Martina Lessio > Postdoctoral Research Scientist > Department of Chemistry > Columbia University > > > On Tue, Apr 24, 2018 at 10:53 AM, Martina Lessio <[email protected]> > wrote: > >> 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 >> > > > > -- > Martina Lessio, Ph.D. > Frontiers of Science Lecturer in Discipline > Postdoctoral Research Scientist > Department of Chemistry > Columbia University > > > _______________________________________________ > 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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