Intrinsic Reaction Coordinate

The path of a chemical reaction can be traced from the Transition State to the products and/or to the reactants, using the Intrinsic Reaction Coordinate method (IRC) [9,10]. The starting coordinates should be a fair approximation of the Transition State. The final values at the endpoint(s) - reactants, products - are computed. The IRC path is defined as the steepest-descent path from the Transition State down to the local energy minimum. The energy profile is obtained as well as length and curvature properties of the path, providing the basic quantities for an analysis of the reaction path. Additional properties along the path (dipole moment, atomic charges) are computed.

Technically speaking the path is computed by taking small steps along the path meanwhile optimizing all atomic coordinates orthogonal to it so that, like in a Linear Transit run, a sequence of constrained optimizations is carried out. The total number of steps along the path is not known in advance. The maximum number of such steps can be set in input. If the path is not completed in the run, a Restart can be used to finish it. Each of the constrained optimizations in the run is treated as it would be in a Linear Transit run: convergence thresholds, maximum numbers of optimization iterations et cetera are set with subkeys in the geometry block.

You can set the IRC runtype by typing it in the geometry block

GEOMETRY
 IRC {Forward} {Backward} {Points=Points} {Step=Step} {StepMax=StepMax}
     {StepMin=StepMin} {Start=Start}
End

IRC

The runtype
IntrinsicReactionCoordinate would also be recognized.

Forward, Backward

Specifies execution of the two possible paths from the Transition State to the adjacent local minima. By default both are computed. If Forward is specified only, the other path is turned off and similarly for Backward. For the definition of which of the two directions down from the Transition State to an adjacent minimum is 'forward' see below.

Points

The maximum number of IRC points computed in the run, for both paths together and including the initial (TS) central point (as far as applicable). Default 100.

Step

The (initial) step length when proceeding from one IRC point to another along the path. The difference between two geometries, to which the step quantity applies, is measured in mass-weighted coordinates. The default value for step is 0.2 (amu)1/2 bohr. Larger steps reduce, in principle, the required number of IRC points from the transition state to the minimum, but usually at the expense of more optimization steps at each of the points so the net gain in computation time may not be very large, or even negative.
The default size is rather conservative and in many cases you may increase it to save a few steps. However, to some extent you can leave that to the program. When going from one point to the next, the program will increase or decrease the stepsize depending on whether or not the previous point to a large number of geometry cycles to converge. The adjusting algorithm also tends to be more cautious when the successive IRC points show more drastic changes in the atomic geometrical configuration. In all cases the IRC step sizes remain between pre-set maximum and minimum values, see the next items.

StepMax

The maximum step length that the program will select in the step-adjusting algorithm. Default: 1.0 or 10 times the initial step length, whichever is larger.

StepMin

The minimum step length that the program will select in the step-adjusting algorithm. Default: 0.03 or 0.3 times the initial step length, whichever is smaller.

Start

Defines how the initial direction of the path is chosen to move away from the Transition State. It does not imply whether the first step along this direction is taken positively or negatively. See for this aspect the section about Forward/Backward IRC paths.
The admissible values for start are:
Grad: compute the gradient and take that direction right from the start. Obviously, if we start perfectly at the Transition State this will be meaningless since the gradient vanishes there completely.
Read : the initial path direction is read in with the key IRCstart, see the section IRC Start Direction.
Hess n : the initial path coincides with the n-th Hessian eigenvector (ordered by ascending eigenvalues); n must be an integer in the appropriate range.
The default (omission of any start specification at all) is the first Hessian eigenvector, presumably corresponding to the path over the Transition State (negative Hessian eigenvalue!).