Direct Evaluation of the Phase Diagrams of Dense Multicomponent Plasmas by Integration of the Clapeyron Equations

TL;DR

This paper introduces an efficient Monte Carlo-based Clapeyron integration method for accurately determining phase diagrams of dense multicomponent plasmas, crucial for modeling stellar interiors, overcoming previous computational limitations.

Contribution

The authors develop a novel, accurate, and efficient method for calculating phase diagrams of dense multicomponent plasmas using direct numerical integration of Clapeyron equations, avoiding finite-size effects and extensive simulations.

Findings

Successfully calculated the melting curve of dense C/O plasmas relevant to white dwarf cores.

Demonstrated the method's accuracy and efficiency over traditional techniques.

Extended the Clapeyron integration approach to electron-ion plasmas.

Abstract

Accurate phase diagrams of multicomponent plasmas are required for the modeling of dense stellar plasmas, such as those found in the cores of white dwarf stars and the crusts of neutron stars. Those phase diagrams have been computed using a variety of standard techniques, which suffer from physical and computational limitations. Here, we present an efficient and accurate method that overcomes the drawbacks of previously used approaches. In particular, finite-size effects are avoided as each phase is calculated separately; the plasma electrons and volume changes are explicitly taken into account; and arbitrary analytic fits to simulation data are avoided. Furthermore, no simulations at uninteresting state conditions, i.e., away from the phase coexistence curves, are required, which improves the efficiency of the technique. The method consists of an adaptation of the so-called Gibbs-Duhem integration approach to electron-ion plasmas, where the coexistence curve is determined by direct numerical integration of its underlying Clapeyron equation. The thermodynamics properties of the coexisting phases are evaluated separately using Monte Carlo simulations in the isobaric semi-grand canonical ensemble. We describe this Monte Carlo-based Clapeyron integration method, including its basic principles, our extension to electron-ion plasmas, and our numerical implementation. We illustrate its applicability and benefits with the calculation of the melting curve of dense C/O plasmas under conditions relevant for white dwarf cores and provide analytic fits to implement this new melting curve in white dwarf models. While this work focuses on the liquid-solid phase boundary of dense two-component plasmas, a wider range of physical systems and phase boundaries are within the scope of the Clapeyron integration method, which had until now only been applied to simple model systems of neutral particles.