Publications / Conference

Calculating Hugoniots for molecular crystals from first principles

Wills, Ann E.; Wixom, Ryan R.; Mattsson, Thomas M.

Density Functional Theory (DFT) has over the last few years emerged as an indispensable tool for understanding the behavior of matter under extreme conditions. DFT based molecular dynamics simulations (MD) have for example confirmed experimental findings for shocked deuterium, enabled the first experimental evidence for a triple point in carbon above 850 GPa, and amended experimental data for constructing a global equation of state (EOS) for water, carrying implications for planetary physics. The ability to perform high-fidelity calculations is even more important for cases where experiments are impossible to perform, dangerous, and/or prohibitively expensive. For solid explosives, and other molecular crystals, similar success has been severely hampered by an inability of describing the materials at equilibrium. The binding mechanism of molecular crystals (van der Waals forces) is not well described within traditional DFT. Among widely used exchange-correlation functionals, neither LDA nor PBE balances the strong intra-molecular chemical bonding and the weak inter-molecular attraction, resulting in incorrect equilibrium density, negatively affecting the construction of EOS for undetonated high explosives. We are exploring a way of bypassing this problem by using the new Armiento-Mattsson 2005 (AM05) exchange-correlation functional. The AM05 functional is highly accurate for a wide range of solids, in particular in compression. In addition, AM05 does not include any van der Waals attraction, which can be advantageous compared to other functionals: Correcting for a fictitious van der Waals like attraction with unknown origin can be harder than correcting for a complete absence of all types of van der Waals attraction. We will show examples from other materials systems where van der Waals attraction plays a key role, where this scheme has worked well, and discuss preliminary results for molecular crystals and explosives.