FreeBird.jl: An extensible toolbox for surface free energy calculation via nested sampling and beyond

The Bayesian-inference-based nested sampling algorithm has recently shown great promise in modeling phase equilibria of adsorbates on solid surfaces at arbitrary temperatures by directly and efficiently calculating the partition function, whose relationship with free energy is well known. In this work, we present FreeBird.jl, an extensible platform for computationally studying phase equilibria on generic interfacial systems, written in the modern programming language Julia. The package supports various system configurations, including atomistic solid surfaces and abstract lattice-gas models, with energies computed via classical interatomic potentials or lattice Hamiltonians. Atomistic and lattice-gas systems can handle single- or multi-component mixtures with arbitrary surface and lattice geometries. Furthermore, we implement other numerical algorithms such as the exact enumeration, Wang-Landau sampling, and Metropolis Monte Carlo. Utilizing Julia's type hierarchies and multiple dispatch features, FreeBird.jl offers a flexible interface that allows users to seamlessly combine system setups, energy calculators, and sampling schemes. Designed for flexibility, extensibility, and performance, FreeBird.jl serves as a comprehensive toolkit for exploring the thermodynamics of interfacial systems and surface phenomena.