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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