[QE-users] Potential issue with charge density cutoff convergence and K-point grid convergence
Martina Lessio
ml4132 at columbia.edu
Wed Apr 25 23:21:49 CEST 2018
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 <degironc at sissa.it>
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 <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>
>> 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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> users mailing listusers at lists.quantum-espresso.orghttps://lists.quantum-espresso.org/mailman/listinfo/users
>
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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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