[Pw_forum] vdw-DF study for calcium decorated graphene

[Janet Wong]

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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