[Pw_forum] Oxygen molecule binding energy

[Stefano de Gironcoli]

Nicola Marzari wrote:
> 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).
>   
I think there's no general consensus whether it's "more correct" to have 
the lowest possible energy giving up the known symmetry of the  true GS 
or to constraint the system to the expected symmetry accepting a 
slightly higher energy... Of course the "true" xc functional would not 
put you in this dilemma, but while awaiting for it, I think the 
important thing is to be aware of the two alternatives and clearly state 
which one one chooses to conpare with.

stefano



> 			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
>>     
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