Finally, note that we performed exploratory DMC calculations using 20 000 walkers and 25 000 steps for the hexamer using the new PES and did not find "holes", i.e., regions of spuriously large negative energies. The ZPE of the cage isomer is roughly 26 cm -1 lower than that of the prism isomer (both ZPEs are referenced to the electronic energy of the prism isomer), in semi-quantitative agreement with the results using q-AQUA, though the uncertainties in this work are much larger due to the smaller number of walkers and steps.