Note that, since the spin crossover of thiophosgene is a reaction in the Marcus inverted regime, we encounter the peculiarity that the imaginary time spent on the product potential becomes negative, i.e. τ < 0. Because the imaginary-time propagator has the form e -τ Ĥg/ (where Ĥg is the nuclear Hamiltonian of the ground electronic state), it is equivalent to a Boltzmann distribution at negative temperature. It is this property which makes the inverted regime so difficult to treat directly using other path-integral methods. However, we recently generalized golden-rule instanton theory for this case 63 by using a trajectory on the product which is traversed backwards in imaginary time.