Recommendations concerning the NEB method.

Preparing input
Please pay attention to the following points when preparing input for a NEB calculation:

• If an approximated transition state is known then try to use this information when preparing the input: do not just specify the initial and final state coordinates in the GEOVAR input section but also add some values in between to take advantage of the higher-order interpolation of the initial reaction path approximation.
• Try to optimize geometries of the initial and final reaction states (the end-points) as good as possible. ADF will by default optimize them too but doing this in advance can save quite a bit of time.
• If you do not want to optimize the end-points during NEB optimization you can use the NoNEBOptEnds input keyword and you know that the end-point geometries do not correspond to local minima, then you must make sure that they lie on the reaction path. If one (or both) of the end-points lie off the path then this may result in the images next to them sliding downhill behind the end-points, which will inevitably break the optimization.
• Choosing an optimization method. There are several optimization methods used in NEB but you should probably use one of the two Quasi-Newton methods: local (default) or global (NEBOPT GlobalQN input option). The main difference in functionality between the wto methods is that the Climbing image technique is possible only with the GlobalQN method. In the default (local) method, the images are optimized in such a way that the cartesian distance between them is always constant. This means that the image with the highest energy is not necessarily the transition state. With the GlobalQN method, you are guaranteed that the image with the highest energy is at the point of the maximum energy on the reaction path.

Problems during optimization
Many problems may be avoided if you follow the recommendations above. If, however, you did follow the recommendations and still have problems then please read below. Here follows a list of common problems with possible solutions:
Optimization stops with a message that the angle has become too small.

• Provided that the end-points are local minima, this may still happen if the initial guess for the reaction path was too rough an approximation. This usually result in very large forces on some of the images, which may result in very large steps. This is not a problem in itself but a problem may be that neighboring images get significantly different steps. If this happens, the NEB chain becomes jagged. This may get quickly out of hands if the Cartesian distances between images are comparable with the steps taken during optimization. The cure in this case is to either reduce the number of images (to increase distances between them) or to decrease the max step size (the STEP RAD= parameter).
• Another reason for the angle becoming too sharp is that the reaction path is very complex. In this case, it may help to use more images to "smoothen" it.

In all other cases it is recommended to contact the SCM support and specify exactly what went wrong and send along the input and output files. It is recommended to use the DEBUG NEB input keyword, which produces extra debugging information. Doing so will speed things up a lot because we won't have to repeat your calculation.