Dear sir, Thank you for your detailed explanation. My understanding on the TS is improved.
best vega On 09/30/10 16:21, Stefano Baroni wrote: > > 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 > > --- > Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste > http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / > stefanobaroni (skype) > > La morale est une logique de l'action comme la logique est une morale > de la pens?e - Jean Piaget > > Please, if possible, don't send me MS Word or PowerPoint attachments > Why? See: http://www.gnu.org/philosophy/no-word-attachments.html > > > > > > > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum -- ================================================ Vega Lew ( weijia liu) Graduate student State Key Laboratory of Materials-oriented Chemical Engineering Nanjing University of Technology, 210009, Nanjing, China *************************************************************** Email: vegalew at gmail.com Office: Room A705, Technical Innovation Building, Xinmofan Road 5#, Nanjing, China *************************************************************** -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20100930/64c2d3b6/attachment.htm
