  In Part I [J. Chem. Phys. 138, 084108 (2013)] we derived a quantum
transition-state theory by taking the t->0+ (short-time) limit of a new form of
quantum flux-side time-correlation function containing a ring-polymer dividing
surface. This t->0+ limit appears to be unique in giving positive-definite
Boltzmann statistics, and is identical to ring-polymer molecular dynamics
(RPMD) TST. Here, we show that quantum TST (i.e. RPMD-TST) is exact if there is
no recrossing (by the real-time quantum dynamics) of the ring-polymer dividing
surface, nor of any surface orthogonal to it in the space describing
fluctuations in the polymer-bead positions along the reaction coordinate. In
practice, this means that RPMD-TST gives a good approximation to the exact
quantum rate for direct reactions, provided the temperature is not too far
below the cross-over to deep tunnelling. We derive these results by comparing
the long-time limit of the ring-polymer flux-side time-correlation function
with that of a hybrid flux-side time-correlation function (containing a
ring-polymer flux operator and a Miller-Schwarz-Tromp side function), and by
representing the resulting ring-polymer momentum integrals as hypercubes.
Together with Part I, the results of this article validate a large number of
RPMD calculations of reaction rates.
