On Sun, 2011-11-06 at 18:33 -0500, Vic Bermudez wrote: > Greetings All, > > I'm struggling with a NEB calculation, and I wonder if someone can > advise > me. I first converged the path using NEB without CI. This took many > iterations (810 to be exact), but it got there in the end. I then restarted > with the optimized path and CI set to 'auto'.
No don't do that. First use the no-CI, just to "stabilize" the path trajectory to some extent, and to get a rough idea on the reaction path. So there is a question: "how do I know when it is stabilized enough". At the beginning of NEB run, the highest energy image can change from iteration to iteration, but when this gets stable, i.e., a particular image displays the highest energy over several number of iterations, say 10-15, then you can stop the no-CI run, and start with the CI run. Just as a rough guide, from my experience, I would say that when the error is on the order of 0.5 eV/A, this is already good enough to plug in the CI. > As you can see from the > attached output, the convergence is extremely slow. In this case I know what > the "right" answer is because I'm reproducing results in the literature. The > forward activation energy should be about 1.65 eV, and at the current rate > at which the activation energy is increasing it will take many thousands of > iterations to get there. Well I don't think this is the reason. Your convergence is already quite good, with almost all the images below 0.05 eV/A. Hence I don't think it will ever climb to 1.65 eV. There must be some difference with respect to what you are comparing to: either the initial state (IS) is different or you located a new TS with lower energy. However in your energy path, I see a lot of problems. As for intermediate images, two of them appear as minima (3 and 6) and two of them as TS (5 and 8). Hence your IS is not really the best minimum (if you calculate the barrier wrt image 3, you get a value of 1.50 eV, which is already closer to your reference value). This is also the primary reason for bad convergence: the NEB convergence is best at describing only one elementary reaction, i.e., IS-->TS-->FS, involving only one saddle point. Please decompose your path into several separate calculations: 1. first do two optimizations by taking the structure 3 and 6. Structure-3 will definitely result in lower energy structure than your IS. 2. when you understand #1, then construct a new NEB run so that it consists of only one elementary step (i.e. one saddle point). Regards, Tone -- Anton Kokalj J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia (tel: +386-1-477-3523 // fax:+386-1-477-3822) Please, if possible, avoid sending me Word or PowerPoint attachments. See: http://www.gnu.org/philosophy/no-word-attachments.html
