Definiation of Extrema in "exts.dat"

jameschen · Post by **jameschen** » Thu May 10, 2007 1:27 am

Hi everyone,
I obtained the four extrema.
The details of the "exts.dat" list in the following.

Extrema 1 found at image 0.000000 with energy: 0.000000
Extrema 2 found at image 0.998673 with energy: 5.278743
Extrema 3 found at image 1.186320 with energy: 5.274689
Extrema 4 found at image 8.008226 with energy: 14.981895

I am not sure that the exact meaning of the "extrema." Is it the energy barrier along the MEP? or It is just a local minimum/maximum along the path.

What does image "0.998673" and "1.186320" mean?
Does those mean image 1?

Thanks for any help and suggestions

James

Post by **graeme** » Thu May 10, 2007 1:47 am

The extrema are the minima and maxmima on a cubic spline between the NEB points. This spline is a little different from typical splines in that the force is known at each image, so a cubic function can be fit between each pair of points. The extrema are the points at which this cubic function has a zero slope.

In your band, a minimum was found at image 0. Then, it looks like the slope at image 1 was slightly negative, giving rise to a maximum just before the image and a minimum somewhat after. Finally, there is a maximum near image 8. If you view the mep.eps after running nebresults.pl, you should see these reported extrema on the spline.

jameschen · Post by **jameschen** » Thu May 10, 2007 2:27 am

Dear Prof. Henkelman
You are right.
Thank you very much for your clear explanations.
Could I say that there are two energy barriers, 5.27 eV and 14.98 eV, in this MEP?

Post by **graeme** » Thu May 10, 2007 3:14 am

I would be a little careful before taking the extrema near point 1 too seriously. The energy of the points is more trustworth than the forces, and this extrema seem to be based only on the force at the first image. But anyways, try minimizing point 1; if it moves to a local minima, it confirms the spline.