Phase Transitions of Noble Gas Adsorbates on Graphite via Nested Sampling

We have recently demonstrated that nested sampling (NS) can efficiently calculate the configuration integral of a noble gas adsorbed on low-index surfaces of face-centered-cubic crystals. Extending this approach, we apply NS to compute the constant-volume phase transition temperatures for low-coverage argon monolayers adsorbed on graphite. Our calculations are compared with experimental measurements of the temperature dependence of the constant-volume heat capacity. The computed heat capacity exhibits two peaks corresponding to transitions between three distinct adsorbate phases, consistent with experimental observations, indicating that NS can accurately reproduce most phase transitions of noble gas adsorbates on graphite. However, prior studies also report a low-temperature peak absent in our calculations. Monte Carlo simulations suggest this peak corresponds to a orientational phase transition involving slight rotations of quasi-hexagonal clusters of adsorbates relative to the basal-plane lattice of graphite. We attribute the absence of this subtle low-temperature transition in our results to finite-size effects, as the system size required to capture this transition is computationally prohibitive for atomistic NS. This suggests that coarse-grained models may be necessary for NS to accurately and efficiently capture certain types of phase transitions.