# A simple thermodynamic model for the hydrogen phase diagram

## TL;DR

This paper presents a classical thermodynamic model that captures key features of solid hydrogen's phase diagram, offering insights into its structure, melting behavior, isotope effects, and quantum phenomena.

## Contribution

It introduces a simple classical model that explains quantum effects in solid hydrogen, bridging classical and quantum descriptions of the system.

## Key findings

- Reproduces main features of hydrogen phase diagram
- Explains quantum effects via classical model
- Provides insights into melting and isotope effects

## Abstract

We describe a classical thermodynamic model that reproduces the main features of the solid hydrogen phase diagram. In particular, we show how the general structure types that are found by electronic structure calculations and the quantum nature of the protons can also be understood from a classical viewpoint. The model provides a picture not only of crystal structure, but also for the anomalous melting curve and insights into isotope effects, liquid metallisation and InfraRed activity. The existence of a classical picture for this most quantum of condensed matter systems provides a surprising extension of the correspondence principle of quantum mechanics, in particular the equivalent effects of classical and quantum uncertainty.