Instantons can be located within the ring-polymer formalism using the INSTANTON command:
INSTANTON[, key1=value, key2=value, ...]
These instantons can be used to compute either the thermal reaction rate or the tunnelling splitting between degenerate potential wells. The behaviour can be controlled with the splitting option. It is a good idea to turn symmetry off (using nosym) during the calculation as the ring-polymer beads do not necessarily share the same symmetry operations as the transition state or well minima.