Session H4: TM First Principles Methods III

Merging Kohn-Sham and Orbital-Free DFT Calculations to Extend the LiH Hugoniot to Very High Pressures

Large-scale hydrodynamic simulations of fluids and plasmas under extreme conditions require knowledge of various properties such as the equation of state (EOS), mass diffusion, and shear viscosity. While many approaches exist for the determination of these properties, one of the most accurate employs quantum molecular dynamics (QMD) simulations on large samples of atoms of the various species. Examples include the shock compression of metal hydrides and the mixing of deuterium/tritium (DT) fuel with ablator materials (such as C/H plastics and Be) in inertial confinement fusion capsules. The quantum nature of the electrons is described with two flavors of finite-temperature density functional theory (DFT), namely orbital-based Kohn-Sham (KS) and Orbital Free (OF). EOSs for Lithium Hydride and Lithium 6 Deuteride (Li6D) have been calculated with both KSMD and with OFMD. The shock Hugoniot for Li6D has been determined for temperatures up to 25 eV (5000 GPa) using a KSMD based EOS, and for T$=$ 5 eV and above (up to 10,000 GPa) using an OFMD based EOS. KSMD simulations here have a practical upper limit of T$=$25 eV due to the scaling of the computational work. The OFMD simulations have a lower limit of T$=$5 eV since the OF DFT yields a poor description at low temperatures. The KSMD and OFMD Hugoniots agree well in the region of overlap (T$=$5 to 25 eV). Comparisons will be presented with experimental data and with shock Hugoniots constructed from both existing EOS tables and from a new, improved SESAME table. By utilizing the KSMD and OFMD results to guide the parameter choices, the new EOS overall is a better match to melt and shock experimental data. This work was performed in collaboration with L. A. Collins, S. Crockett, M. P. Desjarlais, and F. Lambert and under the auspices of an agreement between CEA/DAM and NNSA/DP on cooperation on fundamental science. LANL is operated by LANS, LLC for the NNSA of the USDoE under contract no. DE-AC52-06NA25396.   

First-principles entropy calculations for liquid metals and warm dense matter

The total entropy is not an explicit or easily accessible quantity in first-principles molecular dynamics simulations. It is, however, an extremely important quantity for the calculation of total free energies and derived properties such as equilibrium phase boundaries. In shock experiments the entropy of the shock state determines the release isentrope. Recent advances in the calculation of the entropy for liquid metals and warm dense matter directly from the velocity history in quantum molecular dynamics simulations are presented. The method, a generalization of the 2PT method for classical molecular dynamics [Lin, {\it et al.}, J. Chem. Phys. 2003], significantly increases the accuracy of the method for systems with electronic entropy, spin degrees of freedom, and the softer interactions characteristic of liquid metals and warm dense matter. The results are compared to data and the results of indirect methods, such as coexistence simulations to determine phase boundaries. \vskip .2 in \noindent *Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Density Functional Theory Investigation of Sodium Azide at High Pressure

Sodium azide is being investigated as a potential precursor to a high-nitrogen content energetic material. Changes in the experimentally measured raman spectra under compression and high temperature indicate that a structural change may have taken place. Accurate mode assignments of new peaks arising in the raman spectra have been inconclusive. In this work, the first order raman spectra of sodium azide's alpha and beta phases are calculated using Density Function Pertubation Theory (DFPT) under compression and expansion. Normal mode assignments are made and compared to experiment. In addition, the equation of state of both phases is obtained up to 90 GPa.   

Finite temperature orbital-free density functional theory MD for warm dense matter systems

Warm dense systems present significant challenges for ab initio simulations. In order to incorporate the quantum nature of these systems, state of the art approaches use Kohn-Sham DFT. The number of orbitals required in this approach however scales with the temperature, making it computationally prohibitive. In recent years attention has been given to orbital-free (OF) DFT, which depends only on the density and does not suffer the same scaling issue. For the most part the finite temperature Thomas-Fermi approximation has been used, with some efforts making use of the gradient expansion, or a generalized gradient form. Interestingly, zero temperature OF DFT efforts have made use of non-local functionals based upon linear response theory, with marked improvement over gradient methods. Here we explore extension of this approach to finite temperature. We have implemented the new approximations in both an average atom scheme and for extended systems in OF MD, and will present here the results for system in the warm dense matter regime.