Several Hybrid Monte Carlo-replica exchange (HMC) simulations are carried out to obtain the classical thermodynamic behavior of the ammonia dodecamer and hexadecamer. We find evidence of solid to solid phase changes that we characterize using the structures of low lying minima. In a recent publication [40], we introduce a new integrator for clusters of rigid tops that works directly with non-Cartesian coordinates. This letter contains the first application of the integrator to enhance the efficiency of the Hybrid Monte Carlo simulation of clusters of rigid tops. (C) 2015 Elsevier B.V. All rights reserved.