[ase-users] vdW functional in GPAW program Pt-O dimer

[Thomas Bligaard]

Hi Aleksander,

I think there is not necessarily anything wrong - the vdW-functional can contain 
an (essentially) arbitrary constant. The energies only make sense if you look at 
energy differences between two systems containing the same number of each kind 
of atom (e.g. PtO - 1/2O2 - Pt).

Best wishes,

Thomas


Dawid, A. wrote:
> Hi all GPAW-users
> 
> I have run very simple script to calculate the Van der Waals correction in non-self consistent mode.
> 
> from ase import *
> from gpaw import GPAW
> from gpaw.vdw import FFTVDWFunctional
> 
> a = 6.  # Size of unit cell (Angstrom)
> c = a / 2
> 
> # Pt-O molecule:
> d = 2.1  # Bond length
> molecule = Atoms([Atom('O', (c - d / 2, c, c),magmom=1),
>                   Atom('Pt', (c + d / 2, c, c))],
>                        cell=(a, a, a), pbc=True)
> 
> calc = GPAW(nbands=18, h=0.2, xc='PBE', kpts=(8,6,1),
>              spinpol=True,
>              convergence={'energy': 100,
>                           'density': 100,
>                           'eigenstates': 1.0e-9,
>                           'bands': 'occupied'})#, txt='CO.txt')
> 
> vdw = FFTVDWFunctional(nspins=2,
>                            Nalpha=20, lambd=1.2,
>                            rcut=125.0, Nr=2048,                         
>                            size=None)
> 
> molecule.set_calculator(calc)
> e2 = molecule.get_potential_energy()
> calc.write('PtO.gpw')
> vdW_dif = calc.get_xc_difference(vdw)
> vdW_energy = e2 + vdW_dif
> 
> print 'PtO molecule energy:    %5.5f eV' % e2
> print 'vdW oxygen molecule energy: %5.5f eV' % vdW_energy
> print 'vdW dif energy: %5.5f eV' % vdW_dif
> 
> The result:
> 
> PtO molecule energy:    -7.19200 eV
> vdW oxygen molecule energy: -116.53280 eV
> vdW dif energy: -109.34080 eV
> 
> This energy seams to be quite huge ;-). Have you any idea what I do wrong with that ?
> 
> Best regards
> Aleksander Dawid
> 
> e-mail: A.Dawid at tue.nl
> Technical University Eindhoven
> 
> 
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-- 
-----------------------------------------------
Thomas Bligaard, Ph.D.
Associate Professor
Center for Atomic-scale Materials Design
Department of Physics, Build. 307
Technical University of Denmark
DK-2800 Kgs. Lyngby
Denmark

WWW:   http://www.fysik.dtu.dk/~bligaard/
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