# Presentation: 479263

## 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
- 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

- 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

- 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

- Combust. Theor.
- Mod. **6**, 103 (2002)

- JCP **119**, 7417 (2003)** **

- MD

- QC

## Coefficients of Thermal Expansion of HMX

******300-413**** = 0.00027 K****-1**

******300-450**** = 0.00013 K****-1**

******300-420**** = 0.00020 K****-1**

**private communication (2004)**

**Energetic Matls. Pt. 1, Politzer & Murray Eds.**
**(Elsevier, Boston, 2003), Ch. 10, p. 279**

- 

- 

- 

## HMX thermal conductivity & specific heat

- J. Energetic Matls. **17**, 1 (1999)

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

## Melt curve for HMX

- The melt curve, Tm=Tm(P) is unknown

**Shock T, Hayes EOS**

**T****m****=T****m0****+bP**

**T****m****=T****m0****(1+a********V/V****0****)**

- Combust. Theor.
- Mod. **6**, 103 (2002)

- 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)

## These things matter at the mesoscale...

**J. Reaugh, **
**UCRL-ID-150388 (2002)**

**R. Menikoff, SCCM03**

***At the 2004 Shock Modeling Conference, ***
***the need for good, unreactive EOS***
***information was repeatedly emphasized.***

**If one cannot predict temperature, stress, and their spatio-temporal distributions, s/he cannot credibly predict initiation.**

## ...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

## PETN cold curve, PBE/6-31G**(3-D, all-electron, density functional quantum chemistry)

- 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 -HMX underway (much more complicated system)

- E & P –vs- compression

- Linear compressibilities

## MD Simulations of shocks in crystalline HE

**MD Simulations of shocks in crystalline HE**

## 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

- 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**