[QE-users] Underestimated Adsorption Energy with optB88-vdW

[Gabriel Bramley]

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/users@lists.quantum-espresso.org/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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