PATHSAMPLE is the OPTIM driver program to implement a discrete path sampling (DPS) construction of stationary point databases and perform kinetic analysis.[1,2,3,4,5] This implementation provides a number of different methods for growing stationary point databases in order to produce samples that are kinetically relevant. The main difference from the old version PATHSAMPLE.2.0 is that the geometrical test for distinguishing stationary points has been changed to use the minimum distance. This usage is consistent with OPTIM and the tolerance is set using the keyword GEOMDIFFTOL. Subsequent enhancements of the code are not implemented in PATHSAMPLE.2.0.
PATHSAMPLE relies upon OPTIM  to perform geometry optimisation, especially double-ended searches for pathways between specified minima. In the simplest case, all the OPTIM calls will be Dijkstra connect runs  (this is the case if the CONNECTIONS parameter is less than or equal to one, see §4). Please refer to the OPTIM documentation for full details of this program .
The DPS technique is designed to calculate rate constants between two different regions (A and B) on the potential energy surface, where a region is defined by a set of local minima.[1,2,3,4,5,8,9,10] In PATHSAMPLE.2.0 and above, this objective is achieved by building up a stationary point database using successive double-ended connection runs between local minima, where connections are chosen according to the minimum distance between the minima and their predicted committor probabilities. Note that that this approach is different from the original philosophy, which focused upon the rates associated with individual discrete paths and created new paths by perturbing old ones.
David Wales 2018-03-24