Publications

Lane, James M.; Grest, Gary S.; Thompson, Aidan P.; Desjarlais, Michael P.; Mattsson, Thomas M.

Abstract not provided.

Mattsson, Thomas M.; Root, Seth R.; Magyar, Rudolph J.; Wills, Ann E.; Shulenburger, Luke N.; Flicker, Dawn G.

Abstract not provided.

Magyar, Rudolph J.; Root, Seth R.; Haill, Thomas A.; Mattsson, Thomas M.; Flicker, Dawn G.

Abstract not provided.

Wills, Ann E.; Mattsson, Thomas M.

Abstract not provided.

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

Abstract not provided.

Wills, Ann E.; Hao, Feng H.; Mattsson, Thomas M.; Wixom, Ryan R.; Cragg, George C.

Abstract not provided.

Mattsson, Thomas M.; Desjarlais, Michael P.; Wills, Ann E.

Abstract not provided.

Carpenter, John H.; Flicker, Dawn G.; Root, Seth R.; Magyar, Rudolph J.; Mattsson, Thomas M.

Abstract not provided.

Physical Review Letters

Root, Seth R.; Magyar, Rudolph J.; Carpenter, John H.; Mattsson, Thomas M.

Abstract not provided.

Magyar, Rudolph J.; Root, Seth R.; Carpenter, John H.; Mattsson, Thomas M.

The noble gas xenon is a particularly interesting element. At standard pressure xenon is an fcc solid which melts at 161 K and then boils at 165 K, thus displaying a rather narrow liquid range on the phase diagram. On the other hand, under pressure the melting point is significantly higher: 3000 K at 30 GPa. Under shock compression, electronic excitations become important at 40 GPa. Finally, xenon forms stable molecules with fluorine (XeF{sub 2}) suggesting that the electronic structure is significantly more complex than expected for a noble gas. With these reasons in mind, we studied the xenon Hugoniot using DFT/QMD and validated the simulations with multi-Mbar shock compression experiments. The results show that existing equation of state models lack fidelity and so we developed a wide-range free-energy based equation of state using experimental data and results from first-principles simulations.

Snow, Clark S.; Schultz, Peter A.; Mattsson, Thomas M.; Zavadil, Kevin R.; Brumbach, Michael T.

Abstract not provided.

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.

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

The difficulty of calculating the ambient properties of molecular crystals, such as the explosive PETN, has long hampered much needed computational investigations of these materials. One reason for the shortcomings is that the exchange-correlation functionals available for Density Functional Theory (DFT) based calculations do not correctly describe the weak intermolecular van der Waals' forces present in molecular crystals. However, this weak interaction also poses other challenges for the computational schemes used. We will discuss these issues in the context of calculations of lattice constants and structure of PETN with a number of different functionals, and also discuss if these limitations can be circumvented for studies at non-ambient conditions.

AIP Conference Proceedings

Mattsson, Thomas M.; Magyar, Rudolph J.

Xenon is not only a technologically important element used in laser technologies and jet propulsion, but it is also one of the most accessible materials in which to study the metal-insulator transition with increasing pressure. Because of its closed shell electronic configuration, xenon is often assumed to be chemically inert, interacting almost entirely through the van der Waals interaction, and at liquid density, is typically modeled well using Leonard-Jones potentials. However, such modeling has a limited range of validity as xenon is known to form compounds under normal conditions and likely exhibits considerably more chemistry at higher densities when hybridization of occupied orbitals becomes significant. We present DFT-MD simulations of shocked liquid xenon with the goal of developing an improved equation of state. The calculated Hugoniot to 2 MPa compares well with available experimental shock data. Sandia is a mul-tiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. © 2009 American Institute of Physics.

Mattsson, Thomas M.; Magyar, Rudolph J.

Abstract not provided.

Mattsson, Thomas M.; Wills, Ann E.

Abstract not provided.

Proposed for publication in the Journal of Chemical Theory and Computation.

Wills, Ann E.; Mattsson, Thomas M.

Abstract not provided.

Journal of Chemical Physics

Wills, Ann E.; Mattsson, Thomas M.

Abstract not provided.

Hjalmarson, Harold P.; Hjalmarson, Harold P.; Shaneyfelt, Marty R.; Schwank, James R.; Edwards, Arthur H.; Hembree, Charles E.; Mattsson, Thomas M.

Mechanisms for enhanced low-dose-rate sensitivity are described. In these mechanisms, bimolecular reactions dominate the kinetics at high dose rates thereby causing a sub-linear dependence on total dose, and this leads to a dose-rate dependence. These bimolecular mechanisms include electron-hole recombination, hydrogen recapture at hydrogen source sites, and hydrogen dimerization to form hydrogen molecules. The essence of each of these mechanisms is the dominance of the bimolecular reactions over the radiolysis reaction at high dose rates. However, at low dose rates, the radiolysis reaction dominates leading to a maximum effect of the radiation.