[Pw_forum] Oxygen molecule binding energy
trambui at u.boisestate.edu
Thu Dec 30 00:14:30 CET 2010
Hope everyone is having a great Holiday. would you please help me out
with this, I'm looking for the "ev.f" that help to calculate the bulk
modulus of a material. But I couldn't find it any where, please let me know
where and how I can find that and use it to calculate for the bulk modulus,
Thank you so much,
On Tue, Dec 7, 2010 at 1:14 AM, Sandro Scandolo <scandolo at ictp.it> wrote:
> In the specific case of the O2 molecule, in a molecular orbital picture
> O2 has two degenerate pi-star HOMO orbitals that need to be filled with
> 2 electrons. The ground state is a triplet S=1 (up,up),
> (up,down)+(down,up),(down,down), the next state is a singlet
> (up,down)-(down,up). A good representation of the triplet should be
> given by the ground state of a spin-polarized calculation. The ground
> state should have nup-ndown=2, and the two unpaired electrons fill the
> two pi-star orbitals (with the same spin). I believe you don't need to
> impose any symmetry to find this ground state. If you force the system
> to have total spin=0 (in a spin-polarized calculation), you should
> obtain the state (up,down) (or its degenerate counterpart (down,up)).
> This state does not satisfy the correct symmetry of the 2-electron wfc,
> so in principle does not correspond to any of the correct many-electron
> states above. However this spurious state can be expressed as a linear
> combination (sum or difference) of one of the triplet states
> ((up,down)+(down,up)) and the singlet state ((up,down)-(down,up)).
> Perhaps its energy corresponds to the average energy of the triplet and
> the singlet??!?
> Best regards
> On 06/12/2010 09:45, Stefano de Gironcoli wrote:
> > 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."
> >>> 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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> Sandro Scandolo -- http:/www.ictp.it/~scandolo
> Pw_forum mailing list
> Pw_forum at pwscf.org
B.S. Materials Science & Engineering
trambui at u.boisestate.edu
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