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 <[email protected]
<mailto:[email protected]>> 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
<[email protected] <mailto:[email protected]>> 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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