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["Anglade Pierre-Matthieu" <anglade@gmail.com>]

Hi,

1) I'm not sure your method for computing the binding energy is
perfect. There may be some flaws here. Althought I'm not an expert on
this matter, I would recommand you to try plotting the Energy vs
interatomic distance curve. I've also been told that van der Waals
interactions are (beacuse of error cancelations) best taken into
acount with LDA.

2) You calculation may not be perfectly set up.  Main points are:
a) you use occopt  3 : is you oxygen metallic?
b) you have really few k points in your cell. Is this really enough?

regards

PMA

On 6/27/06, Aloysius Soon <aloysius@physics.usyd.edu.au> wrote:
> Dear all,
> I've been playing around for quite a while with various
> examples found in the Test_v* and Tut, but I can't seem to
> get the right answer for a simple (although not that simple)
> like O2 molecule in a box. Upon subtraction, the binding
> energy that I've got out of a typical GGA calculation is 4
> to 5 eV per O atom, which is way-off the usual that
> literature or other codes I've used (~3.1 to 3.3 eV). I've
> attached one of the many input files that I've used for the
> atom and molecule. Hope someone could help me out in this.
>
> Many thanks.
>
> == input file for oxygen atom ==
> occopt 3           # Could not converge with default
> tsmear 0.03 eV
> #band  16
> #iscf   17
> #npulayit 3
> nsym  1
> nsppol 2           # Needed for spin-polarized run
> kptopt 0
> kpt    0.25 0.25 0.25
>
> #Definition of the unit cell
> acell   9.0   10.0   11.0 angstrom
> #angdeg  90  90  90
> rprim
>                1.00000000000000    0.00000000000000
> 0.00000000000000
>                0.00000000000000    1.00000000000000
> 0.00000000000000
>                0.00000000000000    0.00000000000000
> 1.00000000000000
> chkprim 0
>
> #Definition of the atom types
> ntypat 1
> znucl  8
>
> #Definition of the atoms
> natom 1
> typat 1
> xred
>              0.00000000000000    0.00000000000000
> 0.00000000000000
>
> #Exchange-correlation functional
> ixc 11             # GGA-PBE
>
> #Definition of the planewave basis set
> ecut         70.0 Ry
>
>
> #Definition of the SCF procedure
> nstep 99
> toldfe 1.0d-12
> #tolvrs 1.0d-8
> diemac 1.5
> diemix 0.5
> ===EOF===
>
> == input file for oxygen molecule ==
> occ 1.0  1.0  1.0  1.0  1.0  1.0  1.0  0.0
>      1.0  1.0  1.0  1.0  1.0  0.0  0.0  0.0
> occopt 2
> nsppol 2
> nband  8 8
> nsym  1
> kptopt 0
> kpt    0.25 0.25 0.25
>
> #Definition of the unit cell
> acell     9.0   10.0   11.0 angstrom
>
> #angdeg  90  90  90
> rprim
>                1.00000000000000    0.00000000000000
> 0.00000000000000
>                0.00000000000000    1.00000000000000
> 0.00000000000000
>                0.00000000000000    0.00000000000000
> 1.00000000000000
> chkprim 0
>
> #Optimization of the lattice parameters
> optcell 0
> ionmov  3
> ntime   99
> dilatmx 1.1
> ecutsm  0.5
> prtgeo  2
>
> #Definition of the atom types
> ntypat 1
> znucl  8
>
> #Definition of the atoms
> natom 2
> typat 1 1
> xangst
>                0.00000000000000     0.00000000000000
> 0.60856624824230
>                0.00000000000000     0.00000000000000
> -0.60856624824230
>
> #Exchange-correlation functional
> ixc 11             # GGA-PBE
>
> #Definition of the planewave basis set
> ecut         70.0 Ry
>
> #Definition of the SCF procedure
> nstep 99
> toldfe 1.0d-8
> diemac   1.5
> diemix   0.5
> ===EOF===
>
>
>
> Best regards,
> Aloysius
>
> --
> Aloysius Soon
> Condensed Matter Theory Group
> School of Physics A28, Room 361
> The University of Sydney, NSW 2006 Australia
> Phone: +61 2 903 65389
> Fax: +61 2 935 17726
> Email: aloysius@physics.usyd.edu.au
> Web: http://www.physics.usyd.edu.au/~aloysius/
> ==============================================
> - CARPE DIEM -
> "Gather ye rosebuds while ye may,
> Old time is still a-flying,
> And this same flower that smiles today,
> To-morrow will be dying."
>                         ROBERT HERRICK
>                         1591-1674
> ==============================================


--
Pierre-Matthieu Anglade