Dear QE users,
I managed to resolve the issue below by switching from 'vdw-df' to 'vdw-df2'.
However, when I compared the binding energy of hydrogen for 'relax' and
'vc-relax', I noticed a considerable difference (vc-relax values were ~40%
lower than relax values) in energy values. In theory, the vc-relax case should
be more accurate, but is that a correct assumption or is it more realistic to
go with the values obtained from the 'relax' case?
Best regards,
Janet Wong
Graduate student
University of Toronto, Canada
________________________________
From: Janet Wong
Sent: Wednesday, June 13, 2012 4:06 PM
To: pw_forum at pwscf.org
Subject: vdw-DF study for calcium decorated graphene
Dear QE users,
I am currently trying to calculate the adsorption energy of hydrogen on calcium
decorated graphene using the van der Waals functional. However, I could not
obtain convergence for my system. Prior to this, I calculated the adsorption
energy of hydrogen on the calcium decorated graphene sheet with pbe
pseudopotential using the 'relax' calculation. The adsorption energy calculated
from this latter case was in agreement with another study conducted on the same
system. Then I used the final coordinates of the relaxed structure, and plugged
it into the new input file for the van der Waals functional case and for some
reason the calcium atom and the hydrogen molecules moved further away from the
graphene sheet and the total energy of the system did not converge. I assumed
that the relaxed structure obtained from the pbe pseudopotential case would
have been similar to the relax structure of the van der Waals functional case.
But this does not seem to be true according to my output file. Does anyone have
any suggestions as to what I did incorrectly and how I should fix this? Please
see below for my input file for the vdW-DF case.
&control
prefix='6C_2Ca_4H2_30'
calculation='relax'
outdir = './out'
pseudo_dir = '/RQexec/wongjane/graphene/pseudo/'
forc_conv_thr=8D-4,
/
&system
ibrav=4,
celldm(1)=8.0529,
celldm(3)=5.8666,
nat=16, ntyp=3,
ecutwfc=30, ecutrho=300
occupations='smearing', smearing='methfessel-paxton', degauss=0.02
input_dft='vdW-DF',
/
&electrons
mixing_mode='local-TF',
mixing_beta=0.3,
diagonalization='cg',
conv_thr=0.000007,
/
&ions
ion_dynamics='bfgs'
/
ATOMIC_SPECIES
C 12.01 C.pbe-van_bm.UPF
Ca 40.078 Ca.pbe-nsp-van.UPF
H 1.01 H.pbe-van_bm.UPF
ATOMIC_POSITIONS crystal
H 0.170968317 0.296757710 0.354542022
H 0.280193454 0.194532821 0.355976475
H 0.678887709 0.805195324 0.357509777
H 0.781228558 0.696564044 0.358999571
Ca 0.500714949 0.500950804 0.272750072
C 0.829674303 0.498485404 0.179808930
C 0.496741650 0.831404416 0.179746906
C 0.163637826 0.165386133 0.179821371
C 0.163804328 0.498448431 0.179774763
C 0.496730502 0.165514945 0.179840143
C 0.829848249 0.831571577 0.179766791
Ca 0.500677466 0.501116785 0.086856861
H 0.170992960 0.296734966 0.005052814
H 0.280192056 0.194484391 0.003625759
H 0.678910890 0.805180269 0.002106836
H 0.781279186 0.696574936 0.000615775
K_POINTS automatic
8 8 1 1 1 1
Thank you all in advance.
Regards,
Janet Wong
Graduate Student
University of Toronto
Canada
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