Paths to which the nudged elastic band converges.

A recent letter to the editor (Quapp and Bofill, J Comput Chem 2010, 31, 2526) claims that the nudged elastic band (NEB) method can converge toward gradient extremal paths and not to steepest descent paths, as has been assumed. Here, we show that the NEB does in fact converge to steepest descent paths and that the observed tendency for the NEB to approach gradient extremal paths was a consequence of implementation errors. We also note that while the NEB finds steepest descent paths, these are not necessarily minimum energy paths in the sense of being a set of points which are minima in the potential energy surface perpendicular to the path. An example is given where segments of steepest descent paths follow potential energy ridges.