Recursive entropy in thermodynamics: expounding the statistical-physics basis of the zentropy approach
The recursive property of entropy is well known in information theory; however, the concept is underutilized in thermodynamics, despite being the field where the concept of entropy originated. The zentropy approach is built on this idea, and it has emerged as a useful framework for describing thermodynamic systems across multiple scales, yet its statistical-physics foundation has not been fully articulated. In this work, we establish that foundation by showing that the recursive property allows us to coarse-grain thermodynamic systems into the most useful groups, and deriving the Helmholtz energy and partition function by maximizing entropy in its recursive form. This derivation clarifies the thermodynamic meaning of so-called "states that depend on temperature" as coarse-grained configurations, and maintains a clear distinction between the physical and statistical aspects of statistical mechanics. We then illustrate the usefulness of the approach through two representative applications: magnetic materials, where configurations are defined by spin arrangements, and liquids, where configurations are defined by nearest-neighbor environments. In both cases, the framework enables physically meaningful coarse-graining and captures emergent behavior arising from probability redistribution among configurations. These results position zentropy as an exact and flexible multiscale framework for thermodynamics and statistical mechanics, particularly for systems that admit a natural hierarchical grouping of states.