Sampling Low-Coverage Configurations of Noble Gases on Graphite

Recent studies have shown that nested sampling (NS) effectively calculates phase diagrams of noble gases on low-index face-centered cubic surfaces. To extend this approach to a different surface structure, we apply NS to sub-monolayer argon on graphite, a well-studied system with hexagonal tiling. I will present our NS calculations of constant-volume phase transition temperatures and assess their accuracy in reproducing adsorbate phase transitions reported in surface science literature. Our heat capacities exhibit two peaks, aligning with past experiments and confirming NS captures major phase transitions. However, a lower-temperature peak observed experimentally was initially absent in our calculations. This discrepancy led us to adopt a complementary approach—including NS, molecular dynamics, and a complete enumeration of classical microstates—to understand its origin and refine NS. Monte Carlo simulations suggest this peak arises from rotational phase transitions. Finite-size effects likely explain its initial absence in NS. To overcome computational limits, we developed FreeBird.jl, an efficient NS implementation, to fully capture all transitions.