[Pw_forum] relaxation and TS

[Stefano Baroni]

On Sep 29, 2010, at 5:32 PM, vega lew wrote:

> Through the relaxation of a oxygen molecule on a surface by QE4.2, it dissociated and adsorbed on the surface. The scf energy of each step has decreased in compared with the previous step until the relaxation completed. Now, can one say that there is not any TS for O2 dissociation here ? 
> 
>> Probably not. The potential energy surface might be not very corrugated and the energy barrier for the dissociation might be not very large, so that the dissociation of O2 could happen in a relax calculation. I think there might be a very minute TS in the dissociation process.

here, I beg to differ. If a state, say B, can be reached from another state, say A, following a path along which the energy always decreases, this means *by definition* that there is *no* energy barrier between A and B. I admit that if the energy barrier is so "minute" as to be smaller that the numerical accuracy of the minimization algorithm, it can escape proper detection. (notice that this can be said of most, if not all, numerical computations of any quantity)

> 
> Or is the reaction spontaneous ? 

It is spontaneous

>> If the total energy of system goes down continuously and the adsorption energy of dissociated state is lower than the one of molecular state, you could say the reaction is favorable. Whether it is spontaneous, it is depend on how large of the energy barrier.

in the reported conditions, the energy barrier is zero

>> If there was a TS in this procedure (dissociation and adsorption of O2) would the optimization algorithm of QE4.2 stop the work or continue ? 

In the presence of an energy barrier, the system would stay stuck in the starting configuration

Stefano
 
> If there is only one bond broken in the process you considered, you could try constrained optimization technique. For you information, please refer to B.Hammer (Hammer, B.; Jacobsen, K. W.; Norskov, J. K. Phys. Rev. Lett. 1992, 69, 1971) or PJ Hu's work. (Alavi, A.; Hu, P.; Deutsch, T.; Silvestrelli, P. L.; Hutter, J. Phys. Rev. Lett. 1998, 80, 3650). Or recent developed method by ZP Liu (J. Chem. Theory Comput. 2010, 6, 1136–1144 )

If I have not misunderstood some important point, no constrained minimization or other advanced techniques are needed here.

Stefano B

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Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
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