[Pw_forum] Oxygen molecule binding energy

[Nicola Marzari]

Dear Fabio,


a wild guess on my favourite topic - it could be the difference between
a spherical and a cylindric atom...

I.e. if the O is in the origin, and there is symmetry, the charge
density will be sphericized, but by imposing nosym=.true., you'll
be left with 2 of the p orbitals doubly occupied (i.e. a cylindrical
atom).

I've never managed to get from anyone, including some of the luminaries
in the fundamentals of DFT, a clear answer to which solution is the 
"correct" one. Note that even with nosym=.true. you ahve the option
of having occupation 2.0 on 2 of the p orbitals, or occupation
1.3333333 on oeach of the 3 p orbitals - the latter solution
would probably be similar to the symmetrized solution (but shouldn't
be identical).

			nicola



On 12/3/10 5:27 PM, elbuesta at icqmail.com wrote:
> Dear all,
>
> I recently evaluated with PW the binding energy of a O2 molecule using
> the regular ultra-soft PP from the webpage. I noticed that the energy
> obtained could be 5.7eV or 6.7eV depending if the flag "nosym=.true." is
> used or not in the energy evaluation of the isolated oxygen atom. I
> would like to know if anyone could help me understand why such a large
> difference is obtained for this system, since for many other systems
> (such as metals) the inclusion of "nosym=.true." for the isolated atom
> do not affect so much (or not at all) the value of the binding/cohesive
> energy.
>
>
> Thanks very much for your time, and all best for all!
>
> Fabio Negreiros
> IPCF - CNR, Pisa - Italia


-- 
----------------------------------------------------------------------
Prof Nicola Marzari    Department of Materials    University of Oxford
Chair of Materials Modelling  Director, Materials Modelling Laboratory
nicola.marzari at materials.ox.ac.uk     http://mml.materials.ox.ac.uk/NM