Tunnelling speeds up the reactions, but the geometric phase effect slows them down again.  This recent project in the group was covered in Chemistry World.

We introduce a new approach for calculating perturbative corrections to the ring-polymer instanton approximation to tunneling splittings (RPI+PC) by computing higher-order terms in the asymptotic expansion in ℏ. Our method improves upon the existing instanton theory approach by using information on the third and fourth derivatives of the potential along the tunneling path to include additional anharmonic effects

We describe an algorithm for learning objects in quantum chemistry such as nonadiabatic coupling vectors, which are double-valued near concial intersections.