reason of symmetry breaking in NEB calculation

[farid320]

Dear fellows,
I am calculating dissociation of CO on Fe promoted Ru surface. I have 66 atoms in this system. Out of which two are of Fe. I am carrying out spinpolarized calculations. The calculations are carried out on above and below the slab of FeRu system to cancel any dipole induced in the surface due to adsorbed atom. So CO dissociation into C and O take place in opposite directions.( e.g., one CO dissociation take place from left to right on above the slab while other CO dissociation take place below the slab from right to left. I want to mention that adosorbed species above and below the surfaces of slab have the symmetric structures in initial and final states. Can this breaking of symmetry in NEB calculation be due to half spin nature of electrons? I have observed that Fe doped Ru is paramagnetic. I think when CO dissociation take place in opposite directions above and below the slab symmetry breaks because electron spin is same for both Fe atom above the slab and below the slab.Can this thinknig hold? If this is not the reason of symmetry breaking what can be the reason?

[graeme]

The NEB should converge to a path that passes over first-order saddle points. In your case, the symmetric dissociation of a CO molecule above and a CO molecule below the slab would be over a second-order saddle with twice the energy of a single dissociation. The NEB should, unless constrained by symmetry, converge to a path in which one of the CO molecules dissociates and then the other one does. In this way, the final path will be over two first-order saddles.

I think you can check for dipole corrections without resorting to making a symmetric slab.

[farid320]

Dear Prof. Graeme,
Thanks a lot for your answer to the question.
With regards,
Farid