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

[Martina Lessio]

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 <degironc at sissa.it>
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
>
>
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-- 
Martina Lessio, Ph.D.
Frontiers of Science Lecturer in Discipline
Postdoctoral Research Scientist
Department of Chemistry
Columbia University
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