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

Martina Lessio ml4132 at columbia.edu
Wed Apr 25 22:11:03 CEST 2018

```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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