Intrinsic Reaction Coordinate Calculations

Intrinsic Reaction Coordinate (IRC) calculations can be used to check that the given transition state is the expected transition state for the reaction of interest. IRC calculations start at a transition state and move downhill in energy along the reaction path toward a minimum of the potential energy surface, calculating a series of points in which all geometric variables orthogonal to the path are optimized. The points should lead towards the reactant and the product structures, but as a fixed number of points is calculated, the end points might not actually be reached.

Jaguar has a variation of the IRC method that can be used for inexpensively verifying the transition state. In this variation, the first IRC point is calculated on either side of the transition state. The constraint to the reaction path is then removed and the geometry obtained from each IRC point is optimized to find a minimum. There is of course no guarantee that the minimum corresponds to the end point of the reaction, as the constraints to the path have been relaxed. However, if the minimum found does not correspond to the expected end point, it is possible that the potential energy surface is more complicated than expected, or that the transition state is not the correct one. In this case, reintroducing the constraint by computing a full IRC path is advisable. The variation can be requested by adding three_pt_irc=1 to the gen section. You should also ensure that the step size in the IRC path is large enough to produce a substantial gradient at the first IRC point, otherwise the subsequent optimization may not produce the desired results.

IRC scans have been implemented in Jaguar using the methods described in Ref. [257]. The implementation includes both IRC and minimum energy path (MEP) calculations. The difference between the two is that the reaction coordinate for the IRC path is mass-weighted, whereas the reaction coordinate for the minimum energy path is not.

IRC calculations can be set up from Maestro or by adding keywords to an input file (see Intrinsic Reaction Coordinate (IRC) Keywords in the Jaguar Input File). To set up an IRC or minimum energy path calculation from Maestro, you must first perform a transition-state calculation and read in the restart file, then click the Tasks button and browse to Quantum Mechanics → Reaction Coordinate.

For information on setting up an IRC job in Jaguar, see IRC Panel.