Inspired by the success of the ring-polymer molecular dynamics (RPMD) method, we derive a transition-state-theory version (RPTST) with a dividing surface which is, in general, conical in ring-polymer space. It is explained why this conical form is a good approximation to the optimal dividing surface and therefore why centroid-based quantum transition-state theories are inaccurate for asymmetric barriers at low temperatures.

The geometry of the ring-polymer transition state is found to describe a finite-difference approximation to the semi-classical instanton trajectory (a classical periodic orbit of length βħ on the inverted potential). Based on this, a new practical method for locating multidimensional instantons is proposed, by computing saddle points on the ring-polymer surface, and a derivation for the reaction rate constant based on the "ImF" premise using the ring-polymer formalism is shown to be far simpler than in previous instanton approaches based on functional determinants. The resulting expression is based only on the ring-polymer potential at the transition-state and its Hessian, and is applied to evaluate the rate in a number of polyatomic systems. We show that a free-energy version of the ImF instanton theory is related to RPTST and thereby provide an explanation for why RPMD produces accurate results for thermal reaction rates in the deep-tunnelling regime and demonstrate how it can be made more efficient and systematically improved. From this, we also explain why RPMD is seen to underestimate the rates of symmetric reactions and overestimate the rates of asymmetric reactions.

We also present a ring-polymer instanton derivation of a theory for calculating tunnelling splittings leading to another new practical method, which owing to its simple form, is easily extended to determine the entire tunnelling-splitting pattern of molecular clusters with two or more degenerate wells. This method is applied to the water dimer, trimer, and octamer, and shown to be in good overall agreement with experiment and to provide a deeper understanding of the tunnelling pathways.