  We determine the liquid-solid phase diagram for carbon-oxygen and
oxygen-selenium plasma mixtures using two-phase MD simulations. We identified
liquid, solid, and interface regions using a bond angle metric. To study finite
size effects, we perform 27648 and 55296 ion simulations. To help monitor
non-equilibrium 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
non-equilibrium 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 predominately 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.
