The intrinsic reaction coordinate (IRC) describes the path a reaction follows as it descends from the transition state (TS) to the reactants and products. IRC calculations are commonly performed to confirm that a proposed TS connects to the expected reactants and products and to gain insight into the reaction mechanism.
IRC calculations start from a converged TS geometry. A Hessian (matrix of mixed second derivatives) is calculated, and an initial step is taken in both the "forward" and "backward" directions along the imaginary frequency, using a specified mass-weighted step size. After each step, the Hessian is updated with the energy and gradient at the new point. Next, a pivot point is selected a half step size away from the latest step along the predicted path; calculating the energy and gradient at this pivot point enables estimation of the fourth derivative along the pathway. An optimal full-size step is then determined; if the bending angle at the pivot is too large (typically >90°), the step size is reduced, and the pivot and full step procedure are repeated. Once an acceptable step has been found, the process of stepping and pivoting is repeated until a local minimum is reached or the maximum number of steps is reached.
To perform an IRC calculation, you must start from an optimized TS geometry. On submission, you can specify a level of theory (ideally the same as that used for the TS optimization), a maximum number of steps, and a step size (in amu1/2·Å). Rowan also supports re-optimization of the input structure, but this option should be used with caution unless the starting geometry is very close to the TS. The default parameters often suffice, but some systems may need smaller step sizes to obtain a smooth IRC (particularly those dominated by proton transfer) or more steps to fully capture the entire pathway (for reactions with large-scale motion).