Equation of state of CH1.36: First-principles molecular dynamics simulations and shock-and-release wave speed measurements

We report the computation and measurement of the equation of state of a plastic with composition CH1.36. The computational scheme employed is density functional theory based molecular dynamics, at the conditions: 1.8 g/cm(3) < rho < 10 g/cm(3), and 4000 K < T < 100 000 K. Experimental measurements are of the shock speeds in a geometry in which the plastic is directly abutting a different material, liquid deuterium, from which release wave behavior in the plastic can be deduced. After fitting our computed pressure and internal energy with a Mie-Gruneisen free energy model, we predict the principal shock Hugoniot and various shock-and-release paths and show that they agree with both recently published laser-shock data and our new data regarding the shock speeds on release. We also establish that, at least in the particular (rho, T) range considered, the equation of state of this complex two-component material is well described by an equal pressure and temperature mixture of pure C and H equations of state with a composition-weighted additive-volume assumption. This observation, together with our fit to the limited-range simulation data, can form the basis for the construction of an accurate wide-range equation of state model for this plastic. Implications for its use as an ablator in inertial confinement fusion capsules are discussed.