Forward / Backward IRC paths

Obviously there are two IRC paths down the transition state: Forward and Backward. We would have liked to chemically define forward and backward by determining (in advance!) which of the endpoints is reactants and which products. This is not well doable in practice. Therefore we define the directions in terms of the initial path vector: select simply the atomic coordinate with the largest (absolute) change in the initial vector and define Forward as the direction in which this coordinate increases and Backward as the direction in which it decreases.