Dear Gabriel,
1/2, are your sims. reproducing *all* of the ref. params? (eg. are
cell parameters the same?)
any missing params. would contribute by adding an *unknown*
magnitude of error...
2/2, my guess would be that your evaluation of dE_a is wrong due to
*different cell sizes*
(basis set superposition error?); this would explain the
systematic character of the deviation
and suggest (pain indeed) to increase cell sizes up to the same
(ie. the largest) of
E_slab+ads, E_slab and E_adsorbant.
+1, your eq. for the heat is recursive (kidding): LHS is to be renamed!
HTH,
t
On Wed, Oct 7, 2020 at 12:19 PM Gabriel Bramley <[email protected]> wrote:
>
> Dear Users,
>
> Problem:
>
> I am performing calculations with the optB88-vdW functional to obtain
> adsorption energies of small aromatic molecules on metallic surface facets
> (Pt(111) and phenol for the cases discussed here). Unfortunately, the
> adsorption energies obtained from these calculations are systematically 50 kJ
> mol^-1 below comparable studies performed with VASP. I have replicated their
> parameters as closely as possible - as such I am also using PBE PAW
> pseudopotentials from the psl pseudopotential library.
>
> The above underestimation occurs for a fully periodic calculation. Using the
> dipole corrections (2D and ESM (bc1)) appears to exacerbate these issues,
> leading to a 70 kJ mol^-1 underestimation of the adsorption energy. (The
> system is centred in the centre of the cell with 8 Ang of vacuum either side
> of the system for 2D, and the system is centred around z=0 for ESM).
>
> The calculation for adsorption energy is performed as:
> E_ads = E_slab+ads - (E_slab + E_ads)
> Where the adsorbate is calculated in a 30x30x30 Ang cell with the
> Markov-Payne dipole correction (Though this give negligable difference
> compared to full PBC).
>
> Question:
>
> The PBE+rvv10 functional performs significantly better with PBC and 2D
> Coulomb Cutoff, giving sensible and expected values for the adsorption
> energy. However, I am cautious in trusting these results given the large
> errors for the optB88-vdW functional. Is there something obvious I am missing
> in these calculations or is this a known issue?
>
> A previous email to this list has pointed out other issues with this
> functional
> (https://www.mail-archive.com/[email protected]/msg35965.html).
> This might help, but could someone please clarify this as well?
>
> Additional information:
>
> I am using QE v.6.5 compiled with Intel-2018 with the ELPA 2017 library.
> Enivron Module installed but not used in these calculations. All QC tests
> pass without error.
>
> Please find below the input file:
>
> ___________________________________
>
> &CONTROL
> calculation = 'relax'
> restart_mode = 'from_scratch'
> prefix = 'Pt'
> disk_io = 'low'
> pseudo_dir = '/home/gab1u17/pslib-kj-paw/'
> nstep = 300
> /
> &SYSTEM
> ibrav = 0
> ecutwfc = 45
> ecutrho = 360
> occupations = 'smearing'
> degauss = 0.05
> smearing = 'gaussian'
> input_dft = 'vdw-df-obk8'
> assume_isolated = '2D'
> ntyp = 4
> nat = 48
> /
> &ELECTRONS
> electron_maxstep = 150
> conv_thr = 8e-08
> mixing_mode = 'local-TF'
> mixing_beta = 0.2
> /
> &IONS
> ion_dynamics = 'bfgs'
> /
> &CELL
> /
>
> ATOMIC_SPECIES
> Pt 195.084 Pt.pbe-n-kjpaw_psl.1.0.0.UPF
> C 12.011 C.pbe-n-kjpaw_psl.1.0.0.UPF
> O 15.999 O.pbe-n-kjpaw_psl.1.0.0.UPF
> H 1.008 H.pbe-n-kjpaw_psl.1.0.0.UPF
>
> K_POINTS automatic
> 6 6 1 0 0 0
>
> CELL_PARAMETERS angstrom
> 7.42718991274627 0.00000000000000 0.00000000000000
> 3.71359495637314 6.43213514316980 0.00000000000000
> 0.00000000000000 0.00000000000000 29.000000000003126
>
> ATOMIC_POSITIONS angstrom
> Pt 2.5641489000 0.4594382200 8.0000000000 0 0 0
> Pt 5.2167167200 1.3783146700 8.0000000000 0 0 0
> ...
> Pt 4.1556895900 0.1531460700 17.1683222700
> Pt 6.8082574200 1.0720225200 17.1683222700
> Pt 2.0336353300 1.9908989700 17.1683222700
> ...
> H 0.3060672802 1.6388023741 19.1376040000
> H 2.7167357186 1.0284278006 19.1376040000
> H 4.4562891110 2.8078620154 19.1376040000
>
> ___________________________________
>
> Thank you in advance for your assistance.
>
> Kind regards,
> Gabriel Bramley
> PhD. Candidate
> University of Southampton
>
> _______________________________________________
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> users mailing list [email protected]
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_______________________________________________
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