[QE-users] Potential issue with charge density cutoff convergence and K-point grid convergence
Martina Lessio
ml4132 at columbia.edu
Wed Apr 25 23:30:27 CEST 2018
Dear Jia,
Thanks so much for your feedback. I see your point, although that is the
result I got from my convergence tests. Perhaps I will have another look at
my tests and check that I have not been too conservative with selecting the
parameters. Furthermore the suggested minimum cutoff for one of the pseudo
potentials I am using is 50 Ry so I don't think I can go much lower than
that.
All the best,
Martina
On Wed, Apr 25, 2018 at 5:23 PM, Jia Chen <jiachenchem at gmail.com> wrote:
> Dear Martina,
>
> If I may interject, the advantage of ultrasoft pseudopotentail is not
> using wavefunction cutoff as large as 55 Ry. Usually ultrasoft and norm
> conserving pseudopotential have similar density cutoff, but ultrasoft only
> need about half the wavefunction cutoff.
>
> Cheers
> Jia
>
> On Wed, Apr 25, 2018 at 4:11 PM, Martina Lessio <ml4132 at columbia.edu>
> 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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>
>
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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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