Quantum Enhancement of Microcanonical Negative Heat Capacity Features in the Melting of Water Clusters

For finite systems away from the thermodynamic limit, regions of negative slope in the microcanonical caloric curve can occur, as characterized previously for classical systems. Here, we examine water clusters described by the polarizable MB-pol potential, using classical and quantum harmonic superposition partition functions. With databases exceeding 800 000 local minima for (H2O)20, we characterize premelting and melting in terms of hydrogen-bond statistics, occupation probabilities, and the associated information entropy. We show that heat capacity features can be assigned to specific local minima in the microcanonical ensemble, extending this analysis to entropy. Quantum effects influence the main canonical melting features only weakly but strongly suppress premelting. In contrast, the water octamer develops a clear convex intruder in the quantum microcanonical caloric curve that is absent classically, demonstrating that ensemble inequivalence can arise from quantum effects.