Keyword category:

Transition State Location

IRC=n.n

Description

An IRC calculation follows the reaction vector in much the same manner as PATH. (A more detailed description of the IRC algorithm can be found in Chapter 4, Computational Procedures.) Geometries and properties will be reported at regular intervals along the reaction coordinate. When IRC is specified alone, the default spacing between reported points is chosen based on the total number of degrees of freedom, N, of the system (default spacing = min(0.08*sqrt(N),1.5) Angs). Use IRC=n.n to set this spacing between these reported points to n.n Angstroms.

IRC differs from PATH in that it runs weighted Cartesian coordinates rather than internal coordinates. Since PATH runs in internal coordinates, it can utilize symmetry constraints, whereas IRC can not. IRC also relies on computing second derivatives and so is more reliable but also more expensive then PATH. Because IRC (unlike PATH) uses the Hessian, IR frequencies and THERMO properties can be computed at each point along the IRC.

See also:

PATH or T.V.