quantum-espresso/NEB/examples/example01

This example shows how to use neb.x to calculate the minimum energy path (MEP) 
of a simple activated reaction i.e. the collinear proton transfer reaction :

H2 + H  <==> H + H2

The MEP is obtained by means of the Climbing-Image Nudged Elastic Band (CI-NEB)
method and two different climbing image algorithms are used ("auto" and
"manual").

(for the meaning of the cited input variables see the Doc/INPUT_NEB* files)

The symmetric reaction path H2 + H  <==> H + H2 is calculated in three 
different ways.

1) The path connecting the initial and the final configurations is discretized
   with an odd number of images (7) so that the standard CI_scheme ("auto")
   will give rise to a symmetric MEP (3 images on the left of the saddle point
   and 3 images on the right). Note that in this system the use of the climbing
   image is not necessary. Indeed using CI_scheme = "no-CI" the result is the 
   same.

2) The path connecting the initial and the final configurations is discretized
   with an even number of images (8) and no climbing image is used. The
   resulting path is symmetric, but no image is at the saddle point.

3) The path connecting the initial and the final configurations is again 
   discretized with an even number of images (8), but the "manual" CI_scheme 
   is used so that the resulting path is asymmetric. The image 5 now is at the
   saddle point.

Given the low accuracy of these calculations (plane waves cut-off and 
thresholds) and the small box employed, these results should not be
compared with experiments or with other "ab initio" calculations.