What is the forces standing for in neb.dat?

[cenwanglai]

I have got a neb.dat read like below:
0 0.000000 0.000000 -0.010838 0
1 0.867271 -0.837088 -1.279897 1
2 1.768503 -1.026423 -1.128849 2
3 2.584773 -0.937171 -2.621561 3
4 3.429818 -0.846114 -0.975741 4
5 4.433803 -1.987181 -2.796546 5
6 5.509804 -2.035966 -3.951533 6
7 6.561177 -2.989551 0.890184 7
8 7.546694 -2.312575 -0.077526 8

(1)What does a positive or a negative force item mean in the fourth column? Do the plus or minus sign directs the movement of the images?

(2)The distance between two nearby images is supposed to be lower than 1 Angstrom?

(3)Can anyone give me some advice about the 7th line with the lowest energy? The 8th line has been relaxed to ground state and is supposed to be CO-adsorbed-slab Final State. And the first line stands for configuration that CO in the vacuum off the slab surface.

[graeme]

It is a little tricky to see what's going on from a list of numbers, but here's a guess.

There does not appear to be a barrier for adsorption on the surface since the energy decreases from image 0->2. We typically use the NEB to find pathways between stable states. There may be a local minimum near image #2, but there is certainly a more stable adsorption geometry near image #7 which should be minimized.

Then, consider what you are trying to calculate. If there is no barrier for adsorption, this band may only tell you that the energy drops as you go from CO in vacuum to CO on the surface. If that is the case, there is no need for finding a minimum energy pathway.

Column 4 is the force projected along the band. A negative force means that the energy is going down along the band and a positive force means that it is going up.

The distance between images can be smaller than 1 Ang. If you used twice the number of images to represent the same band, the distance between images will decrease by a factor of two. In the limit of many images, that distance between images can be arbitrarily small. There are no interatomic interactions between images along the band.

[cenwanglai]

I have relaxed the 7th line CONTCAR and found that it transformed to tht 8th FS.

The INCAR for NEB(just main tag listed):
ENCUT = 380
EDIFF = 1E-3
EDIFFG = -1
IBRION = 3
POTIM = 0
IOPT = 3
CHAIN = 0
ISPIN = 2

The INCAR for relax:
ENCUT = 480
EDIFF = 1E-4
EDIFFG = -0.02
IBRION = 1
POTIM = 0.4
ISPIN = 2


It sounds unaccountably.
Yes. the adorption for CO to slab is not wealthy of NEB running. I just do it do learn NEB calculation and verify the FS.

[graeme]

There is some problem. The neb.dat shows the energy of the 7th image to be lower than the 8th image FS. A relaxation can not increase the energy.