Extrema images in CL-NEB and distances between images

[cenwanglai]

[attachment=0]CNEB.png[/attachment]

I am running a Cl-NEB run and now the nebef.pl results read like above. We can find two extrema in energy: the local minimum of image 2 at -0.0675 and the maximum of image 4 at -0.0299 ref to initial image 00.

When I check the OUTCAR of all the images, I am puzzled:

for 01/OUTCAR:
NEB: the previous image is higher in energy: F
NEB: the next image is higher in energy : F
NEB: image is at an extrema

for 02/OUTCAR:
NEB: the previous image is higher in energy: T
NEB: the next image is higher in energy : T
NEB: image is at an extrema

for 04/OUTCAR:
NEB: the previous image is higher in energy: F
NEB: the next image is higher in energy : F
NEB: image is at an extrema

My question is: for 01/OUTCAR, why "NEB: the previous image is higher in energy: F" ? It should be TRUE! And the image 01 is not an extrema!

The next two extrema are right. And, the MEP will be divided in to three areas: 00-02, 02-04 and 04-07. And the distance bwtween neighbor images will be pushed to be equal for each of the three areas, respectively?

[graeme]

This is a good point; and something that we know about. The logic for this is as follows:

VASP does not read the energy of the first (00) and final (07, in your case); it only reads those geometries.

You can set the energy of the endpoints through the INCAR file using:

EFIRST = -661.829567 (or whatever the initial energy happens to be)
ELAST = -665.160154

and everything should work as you expect. But when there is no information about the energy of the endpoints, we assume that they are minimized and therefore have a lower energy than the next-neighbor images (01 and 06 in your case). If this is not true, we suspect that you are doing something unusual such as finding a path that is not between local minima. If this is the case, we recommend that you turn off the climbing image.

[cenwanglai]

Dear Graeme, you are so helpful! I love you! ~

I put a small molecule in the slab vacuum 4 Angstrom from the surface. The molecule will be adsorbed to the surface and react with pre-adsorbed surface species. In this process, there is a energy barrier.

So the initial geometry is true acting as a guess beginning. But, I think the climbing tage (= TRUE)is still needed. Do you agree my opinion?

When I have set EFIRST and ELAST as you suggested, a new line was added to all the 01-06/OUTCAR :

NEB: no climbing image found.

What does this mean to 04 image which is supposed to be the TS with a local maximum energy? For another CNEB run with two fully optimized endpoints, the same INCAR (except images number) did not make this line. Should I set the EFIRST to a value lower than the 04 image energy but higher than 01 image to make the CLIMBING functional work?

Why did you suggest in my case to turn off climbing tag?

[cenwanglai]

Graeme, the line (NEB: no climbing image found) disappeared when I set the EFIRST to a value between the energy 01 image and 04.

I think I have got your points in EFIRST and ELAST in INCAR.

Cheers ~