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
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