[QE-users] Potential issue with charge density cutoff convergence and K-point grid convergence

[Stefano de Gironcoli]

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 list
> users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users

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