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Atomistic Studies of Fundamental Properties and Processes: C-SAFE & LANL T-14 Interactions

Atomistic Studies of Fundamental Properties and Processes: C-SAFE & LANL T-14 Interactions Thomas D. (Tommy) Sewell Theoretical Division Explosives and Organic Materials Group Los Alamos National Laboratory Motivation Part of an overall upscaling/homogenization modeling methodology

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Atomistic Studies of Fundamental Properties and Processes: C-SAFE & LANL T-14 Interactions

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  1. Atomistic Studies of Fundamental Properties and Processes: C-SAFE & LANL T-14 Interactions Thomas D. (Tommy) Sewell Theoretical Division Explosives and Organic Materials Group Los Alamos National Laboratory

  2. Motivation • Part of an overall upscaling/homogenization modeling methodology • Upscaling: atomisticsmesoscale • Physical quantities in (P,T) space • Increasingly, phenomenology such as plastic dissipation • Homogenization: mesoscalemacroscale • Spatio-temporal distributions of stress, temperature, etc. • the “tails” of the distributions are what matter for initiation • Basis for statistical mechanics-based constitutive description • Guided by needs of, interactions with, mesoscale modelers • C-SAFE, LANL T-14 share many common interests • Synergistic interactions have resulted • Sewell with GSmith and Voth • Bardenhagen with McMurtry and co-workers

  3. Philosophy slide • Simulations: Let the (end) user beware • Is the potential-energy surface validated? • Are the methods sound for the problem? • Are the accessible length and time scales compatible with the property or phenomenon of interest? • Are the results likely to be highly corrupted by pesky quantum effects? • Was care taken to ensure convergence? • Experiments: Don’t accept uncritically numbers from the handbooks as well-determined fact • Must also pay attention to thermodynamic state dependencies of important quantities

  4. Comparison of HMX isotherm and hugoniot data JCP 119, 7417 (2003) • Red: Yoo & Cynn, 3rd-order Birch-Murnaghan fit • Black: Olinger et al., Us-Up fit • Grey diamonds: hugoniot, solvent-pressed • Grey circles: hugoniot, single crystal MD Combust. Theor. Mod. 6, 103 (2002) QC

  5. Coefficients of Thermal Expansion of HMX private communication (2004) b 300-413 = 0.00027 K-1 a 300-420 = 0.00020 K-1 300-450 = 0.00013 K-1 d Energetic Matls. Pt. 1, Politzer & Murray Eds. (Elsevier, Boston, 2003), Ch. 10, p. 279

  6. HMX thermal conductivity & specific heat J. Energetic Matls. 17, 1 (1999) Chem. Phys. Letters 324, 64 (2000).

  7. Melt curve for HMX • The melt curve, Tm=Tm(P) is unknown Depending on the form used for the melt curve, one predicts that HMX will melt in a ~19 GPa shock, or not at all! But see: PRL 92, 235702 (2004) Tm=Tm0+bP Tm=Tm0(1+aDV/V0) Shock T, Hayes EOS Combust. Theor. Mod. 6, 103 (2002)

  8. These things matter at the mesoscale… At the 2004 Shock Modeling Conference, the need for good, unreactive EOS information was repeatedly emphasized. J. Reaugh, UCRL-ID-150388 (2002) R. Menikoff, SCCM03 If one cannot predict temperature, stress, and their spatio-temporal distributions, s/he cannot credibly predict initiation.

  9. …and we are increasingly convinced that: • Given a validated force field, atomistic simulation • will often yield predictions, for appropriate quantities, that are within experimental uncertainties • can be used (with care) to obtain information in thermodynamic regimes where experiments are difficult, if not impossible, to perform • The C-SAFE/GSmith and LANL/T-14 collaboration is developing methods and specific predictions for many properties of interest • This research is gaining increasingly high-profile attention within the mesoscale and homogenization communities at Los Alamos

  10. Two Examples of LANL-Centered Atomistic Studies That Have Profited From C-SAFE/LANL Collaborations

  11. PETN cold curve, PBE/6-31G**(3-D, all-electron, density functional quantum chemistry) E & P –vs- compression Linear compressibilities • Quite good agreement with experiment • Not necessarily expected, especially at low pressure; see J. Phys. Chem. B 108, 13100 (2004) • Phys. Rev. B 69, 035116 (2004) • Analgous studies of b-HMX underway (much more complicated system)

  12. MD Simulations of shocks in crystalline HE

  13. Cutout of snapshot from “large” simulation(cell length along long axis is 0.15 micron) Challenge is to identify, characterize, and quantify the phenomena

  14. Summary • We are pushing hard to gain as much information as possible from atomistic simulations • In many cases, atomistic simulation predictions are within experimental uncertainties • We are now at a point where one can credibly use large-scale MD to study shocks and quasi-static loading to understand dissipation in complicated molecular crystals • The LANL/T-14 C-SAFE/GSmith-Voth collaborations have been, and continue to be, of high value in advancing our capabilities and confidence in these areas

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