[QE-users] Potential issue with charge density cutoff convergence and K-point grid convergence
Stefano de Gironcoli
degironc at sissa.it
Wed Apr 25 23:16:39 CEST 2018
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 <ml4132 at columbia.edu
> <mailto:ml4132 at columbia.edu>> 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
> <degironc at sissa.it <mailto: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
>
>
>
>
> --
> Martina Lessio, Ph.D.
> Frontiers of Science Lecturer in Discipline
> Postdoctoral Research Scientist
> Department of Chemistry
> Columbia University
>
>
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