IRC=1 and IRC=1*
This option is intended for use with a transition state. The path from the transition state to either the reactants or the products is generated. The path generated depends on the coefficients of the eigenvectors of the normal mode. There is no easy a priori way of determining what the phase of these coefficients is, so try running IRC=1, and see if it goes the correct way. If not, use IRC=-1. If IRC=1* or IRC=-1* is used, then the entire reaction path, from reactants or products, through the transition state, to products or reactants, is mapped. If DRC is present, then the system is given an initial perturbation in the positive normal coordinate, then the DRC is calculated.
These two options require the normal mode for the reaction. A FORCE or FORCETS calculation must be run in order to generate the normal mode. As this is computationally expensive, a useful strategy is to run the FORCE or FORCETS calculation and use ISOTOPE to save the results. Then the IRC calculation can be run using, e.g., IRC=1 RESTART. If RESTART is not present, a FORCE calculation will automatically be run first.
IRC=+n or IRC=n
Normally used with DRC, this option perturbs the system along the n'th normal mode. One quantum of kinetic energy is added, the value of the quantum being that of the associated vibrational energy. This option allows the exact normal mode to be mapped out. IRC=n with DRC can be used with both ground and transition state systems. If IRC=1 and DRC is used with transition states, it would map out the dynamic path from the transition state to reactants or products.
The same as IRC=+n, except the initial perturbation is in the opposite direction. Except for transition states, this option can be duplicated by one of the other options described here.
When studying reaction paths involving transition states, one option, although implied logically from the above description of the IRC, should not be used. The option IRC=n, where n is not equal to 1 or -1, is meaningless. To see why, consider the option IRC=1. That maps the reaction path, and uses the lowest transition state normal mode, i.e. the mode with the imaginary frequency. Any other mode would simply return to the transition state, in other words, it would do nothing useful.