  Nucleosynthesis, on the surface of accreting neutron stars, produces a range
of chemical elements. We perform molecular dynamics simulations of
crystallization to see how this complex composition forms new neutron star
crust. We find chemical separation, with the liquid ocean phase greatly
enriched in low atomic number elements compared to the solid crust. This phase
separation should change many crust properties such as the thermal conductivity
and shear modulus. The concentration of carbon, if present, is enriched in the
ocean. This may allow unstable thermonuclear burning of the carbon and help
explain the ignition of the very energetic explosions known as superbursts.
