A "reaction coordinate" is envisioned as a continuous transformation
from reactant-like to product-like geometry along a minimum-energy
pathway, passing through the high-energy transition state
geometry. The *intrinsic reaction coordinate* (IRC)
is a particular choice that corresponds
to infinitely slow passage along a steepest-descent
trajectory from the transition state towards the connecting
reactant or product equilibrium minima,
with complete equilibration (optimization) of all remaining geometrical
degrees of freedom.