Direct molecular dynamics simulation of liquid-solid phase equilibria for two-component plasmas

Abstract

We determine the liquid-solid phase diagram for carbon-oxygen and oxygen-selenium plasma mixtures using two-phase molecular dynamics simulations. We identify liquid, solid, and interface regions using a bond angle metric. To study finite-size effects, we perform 27 648-and 55 296-ion simulations. To help monitor nonequilibrium effects, we calculate diffusion constants D i . For the carbon-oxygen system we find that D O for oxygen ions in the solid is much smaller than D C for carbon ions and that both diffusion constants are 80 or more times smaller than diffusion constants in the liquid phase. There is excellent agreement between our carbon-oxygen phase diagram and that predicted by Medin and Cumming. This suggests that errors from finite-size and nonequilibrium effects are small and that the carbon-oxygen phase diagram is now accurately known. The oxygen-selenium system is a simple two-component model for more complex rapid proton capture nucleosynthesis ash compositions for an accreting neutron star. Diffusion of oxygen, in a predominantly selenium crystal, is remarkably fast, comparable to diffusion in the liquid phase. We find a somewhat lower melting temperature for the oxygen-selenium system than that predicted by Medin and Cumming. This is probably because of electron screening effects.

New full evolutionary sequences of H- and He-atmosphere massive white dwarf stars using mesa

“…Those values where obtained by Potekhin & Chabrier (2010). Results from the asteroseismological analysis performed by Romero et al (2013) indicates that the azeotropic type phase diagram from Horowitz et al (2010) (see also Schneider et al 2012;Hughto et al 2012) better represents the crystallization on white dwarfs cores. Hence, we modified the values for the coupling constant Γ to Γ i = 215 and Γ full = 220 in our computations.…”

Hydrodynamic shock wave studies within a kinetic Monte Carlo approach

“…Many advances have been made in MD calculations providing unique insight into fluid dynamics on molecular scales (see e.g. [57][58][59][60]). However, the one-to-one physical-to-simulated particle ratio generally limits MD calculations to the numerical study of matter in small volumes.…”

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

“…Over the few last decades, several methods have been proposed and used to map the phase diagrams of dense astrophysical plasmas. They can roughly be classified into three categories: density-functional methods [16,17], Monte Carlo-based (MC) techniques [18][19][20][21][22], and molecular dynamics (MD) approaches [15,[23][24][25]. Density-functional techniques, which rely on analytical models of the free energies of the relevant phases are inherently more approximate than MC and MD simulation techniques.…”

“…Two-phase MD methods such as those used by the Horowitz et al group [15,[23][24][25] have the advantage of not requiring any interpolation. Typically, a liquid and a solid phase are initially put in contact and then evolved in time in the microcanonical or canonical ensemble.…”