Bulletin of the American Physical Society
19th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 60, Number 8
Sunday–Friday, June 14–19, 2015; Tampa, Florida
Session P4: Phase Transitions III: Molecular Dynamics |
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Chair: Frank Cherne, Los Alamos National Laboratory, Luke Shulenburger, Sandia National Laboratories Room: Grand H |
Wednesday, June 17, 2015 11:15AM - 11:30AM |
P4.00001: A High Pressure-Temperature Density Functional Tight Binding Model for Carbon with Dispersion Corrections Chris Cannella, Nir Goldman Carbyne (e.g., linear chains of sp-bonded carbon) has been the subject of intense research focus due to its presence in astrophysical bodies, as well as its potential for use as a nanoelectronic device and superhard material. In this work, we discuss the formation of carbyne fiber bundles over a nanosecond time scale in laser pulse melting studies, using a previously determined density functional tight binding model for carbon coupled with a new correction for the dispersion energy. We determine our dispersion energy model by optimizing a modified Lennard-Jones potential to an experimentally determined equation of state for graphite, yielding excellent results for the bulk modulus and density under ambient conditions. We then simulate previous experiments by heating graphite to high temperature, followed by expanding the ensuing liquid phase to low density. Our results indicate that the initial, hot liquid phase mainly consists of sp$^2$-bonded carbon atoms, which form a system of sp-bonded strands bound together via dispersion interactions upon achieving low density and temperature. The high computational efficiency of our approach allows for direct comparison with experiments that span a wide range of thermodynamic conditions, and can help determine parameters for synthesis of carbon-based materials with potentially exotic properties. [Preview Abstract] |
Wednesday, June 17, 2015 11:30AM - 11:45AM |
P4.00002: Molecular Simulation of the Free Energy for the Accurate Determination of Phase Transition Properties of Molecular Solids Michael Sellers, Martin Lisal, John Brennan Investigating the ability of a molecular model to accurately represent a real material is crucial to model development and use. When the model simulates materials in extreme conditions, one such property worth evaluating is the phase transition point. However, phase transitions are often overlooked or approximated because of difficulty or inaccuracy when simulating them. Techniques such as super-heating or super-squeezing a material to induce a phase change suffer from inherent timescale limitations leading to ``over-driving,'' and dual-phase simulations require many long-time runs to seek out what frequently results in an inexact location of phase-coexistence. We present a compilation of methods for the determination of solid-solid and solid-liquid phase transition points through the accurate calculation of the chemical potential. The methods are applied to the Smith-Bharadwaj atomistic potential's representation of cyclotrimethylene trinitramine (RDX) to accurately determine its melting point (Tm) and the alpha to gamma solid phase transition pressure. We also determine Tm for a coarse-grain model of RDX, and compare its value to experiment and atomistic counterpart. All methods are employed via the LAMMPS simulator, resulting in 60-70 simulations that total 30-50ns. [Preview Abstract] |
Wednesday, June 17, 2015 11:45AM - 12:00PM |
P4.00003: Vibrational and Thermophysical Properties of PETN from First Principles Joseph Gonzalez, Aaron Landerville, Ivan Oleynik Thermophysical properties are urgently sought as input for meso- and continuum-scale modeling of energetic materials (EMs). However, empirical data in this regard are often limited to specific pressures and temperatures. Such modeling of EMs can be greatly improved by inclusion of thermophysical properties over a wide range of pressures and temperatures, provided such data could be reliably obtained from theory. We demonstrate such a capability by calculating the equation of state, heat capacities, coefficients of thermal expansion, and Gruneissen parameters for pentaerythritol tetranitrate (PETN) using first-principles density functional theory, which includes proper description of van der Waals interactions and zero-point and thermal free energy contributions to pressure, the latter being calculated using the quasi-harmonic approximation. Further, we investigate the evolution of the vibration spectrum of PETN as a function of pressure. [Preview Abstract] |
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