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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

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
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
philosophy slide
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
comparison of hmx isotherm and hugoniot data
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

coefficients of thermal expansion of hmx
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

hmx thermal conductivity specific heat
HMX thermal conductivity & specific heat

J. Energetic Matls. 17, 1 (1999)

Chem. Phys. Letters 324, 64 (2000).

melt curve for hmx
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)

these things matter at the mesoscale
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.

and we are increasingly convinced that
…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
two examples of lanl centered atomistic studies that have profited from c safe lanl collaborations
Two Examples of LANL-Centered Atomistic Studies That Have Profited From C-SAFE/LANL Collaborations
petn cold curve pbe 6 31g 3 d all electron density functional quantum chemistry
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)
cutout of snapshot from large simulation cell length along long axis is 0 15 micron
Cutout of snapshot from “large” simulation(cell length along long axis is 0.15 micron)

Challenge is to identify, characterize, and quantify the phenomena

summary
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