The instrinsic reaction coordinate is defined according to Fukui et al.[1] as an imaginary minimum energy trajectory which passes through the transition state and moves infinitely slow. Its initial direction at the transition state is given by the normal mode of imaginary frequency.

The traditional method " has been to choose one or two degrees of freedom as independent variables and to optimize or constrain in some manner the remaining degrees of freedom (Geometry Scan) [2]. Paths generated by the above methods are not unique and often can give rise to strange results such as discontinuities in path [3,4].

If one starts at either the reactants or products, the sense of which direction, pointing up in energy, leads to the transition state is quite arbitrary. Thus, one is never assured that such a procedure will lead to the saddle point for the reaction in question [2].

ORCA features its own implementation of Morokuma and coworker's popular method [2]. The IRC method can be simply invoked by adding the IRC keyword as in the following example.