Reactive Force Field &�Molecular Dynamics Simulations �

(Theory & Applications) Ying Li

Collaboratory for Advanced Computing & Simulations Department of Chemical Engineering & Materials Science

Department of Computer Science

Email: yli12@usc.edu

Feb 19th, 2013

Outline

•  Theory: Simulation method: Parallel reactive molecular dynamics (MD)

RMD ReaxFF

•  Applications: 1.  Combustion of aluminum nanostructures: Aluminum

(Al) nanoparticle aggregates & Al nanorods 2.  Shock-induced detonation and associated reaction

pathway on high explosives: RDX

Research Background

•  Objective: Understanding the combustion mechanism of energetic materials by atomistic simulations

•  Challenge: Reactive atomistic simulations •  Large scale (multimillion atoms) •  Long time (nanosecond)

Reactive force field (RMD & ReaxFF)

Reactive MD

Al Nanostructures

RDX TATB

Simulation Method

Reactive Force Fields (RMD)

•  Ceramic Al2O3 (Vashishta et al., 2008)

•  Al EAM Potential (Voter and Chen, 1987)

•  Hybrid Potential for Al-O system

Reactive Force Fields (RMD) •  Ceramic Al2O3 (Vashishta et al., 2008)

•  Metallic Al (Voter and Chen, 1987)

Environmental Dependent Interpolation

Oxidation degree:

Coupling of two potentials

Validation of Al2O3 Potential Cohesive energy, elastic constants and melting

Liquid structure

P. Vashishta et al., J. Appl. Phys. 103, 083504 (2008)

Validation of EAM Potential

Cohesive energy, lattice constant, bulk modulus and elastic constants

A. F. Voter et al., Mat. Res. Symp. Porc. 82, 175 (1987)

Validation of RMD for AlxOy Comparison of bond length and bond angle in AlxOy fragments

W. Wang, PhD Thesis, USC (2008) P. Politzer et al., J. Phys. Chem. A, 105, 7473-7480 (2001)

QM

Reactive Force Fields (ReaxFF)

Classification of ReaxFF Potential

Bonded

Non-bonded

E = Elp + Eover + Eunder + Ebond + Eval + Epen + Ecoa

+ Etors + Econj + Ehbond + EvdWaals + Ecoulomb

•  Coordination, 2-body, 3-body, 4-body, hydrogen bonding, and nonbonding energies

• Reactive bond order : {BOij} → Bond breakage & formation

Bond Order (ReaxFF)

Bond Order

• Reactive bond order potential energy: Ebond({rij},{BOij}) → Bond breakage & formation

Bond Energy

Developed by Goddard, van Duin, et al. (Caltech)

Bond Energy (ReaxFF: Term 4)

Variable Charge Problem (ReaxFF: Term12)

• Charge-equilibration (QEq) → Charge transfer Determine atomic charges {qi | i = 1, ..., N} every MD step to minimize EES(rN,qN) with charge-neutrality constraint: Σi qi = 0

O2 dissociation on Al(111)

ReaxFF: Periodic Table Published ReaxFF Force Fields: - Hydrocarbons (van Duin, Dasgupta, Lorant & Goddard, JPC-A ‘01, 105, 9396; van Duin & Sinninghe

Damste, Org. Geochem. ‘03, 34, 515) - Si/SiO2 (van Duin, Strachan, Stewman, Zhang, Xu & Goddard, JPC-A ‘03, 107, 3803) - Nitramines/RDX/TATP (Strachan, van Duin, Chakraborty, Dasgupta & Goddard, PRL 2003, 91, 09301;

Strachan, van Duin, Kober & Goddard, JCP ‘05, 122, 054502; Han, Strachan, van Duin & Goddard, in preparation; van Duin, Dubnikova, Zeiri, Kosloff & Goddard, submitted to JACS)

- Al/Al2O3 (Zhang, Cagin, van Duin, Goddard, Qi & Hector, PRB ‘04, 69, 045423) - Ni/Cu/Co/C (Nielson, van Duin, Oxgaard, Deng & Goddard, JPC-A ‘05, 109, 493) - Pt/PtH (Jacob, van Duin, Niemer & Goddard, submitted to JPC-A; Chen, Lusk, Kee, van Duin &

Goddard, submitted to JCP) - Mg/MgH (Cheung, Deng, van Duin & Goddard, JPC-A ‘05, 109, 851)

: not currently described by ReaxFF

Validation of Reax-FF

• 1,600 structures & 40 reactions for H, C, N & O by density functional theory (DFT)

• Good agreement with DFT for RDX decomposition pathways

A. Strachen et al., Phys. Rev. Lett. 91, 098301 (2003)

Parallelization •  Entire system is divided

into sub-system and each sub-system is assigned to one processor

•  Each processor calculates force on its resident atoms

•  Neighboring atoms information is copied to calculate the force on the surface

Linked List Cell Normal Traversal: •  Search space: Full system •  Computation: O(N2)

Linked List Cell •  Length of each cell is at least equal to rcut •  Search space: Only neighboring cells •  Computation: O(N)

Scalability of Parallel ReaxFF MD

Parallel efficiency is 0.957 on 786,432 BG/Q cores for ReaxFF on 8,455,716,864 atoms RDX

N

P

Weak scaling N = 10752 P

Outline

•  Theory: Simulation method: Parallel reactive molecular dynamics (MD)

RMD ReaxFF

•  Applications: 1.  Combustion of aluminum nanostructures: Aluminum

(Al) nanoparticle aggregates & Al nanorods 2.  Shock-induced detonation and associated reaction

pathway on high explosives: RDX

Aluminum Nanostructures Experimental Background

Vapor deposited Al-nanorod

C. Li, et al., Chem. Mater. 19, 5812-5814 (2007)

Close packed Al-NP aggregate

Y. Gan et al., Combust. Flame, 158, 354–368 (2011)

Aluminum nanoparticle (Al-NP) core – metallic Al shell – 2~4 nm amorphous Al2O3

Y. A. Kotov et al., Nanotech. in Russia, 4 (2009)

Aluminum combustion reaction

microscale vs. nanoscale

Y. Sun et al., Defense Science J., 4, 56 (2006)

Oxidation of Al-NP Aggregates

•  Questions: 1.  After oxidation, Al-NP agglomerate or fragment? 2.  What’s the size effect on the oxidation of Al-NP aggregates? 3.  What are the reaction pathways (or intermediate products)?

In air

With CuO

Y. Ohkura, et al., Combust. Flame, 158, 2544-2548 (2011)

Small fragments

Large agglomerations

core Al shell Al shell O

Combustion Propagation for D = 26 nm Al-NP Aggregate

Al Nanorods Oxidation

S. K. Cheah et al., Nano Letters 10, 9, 3230-3233 (2009)

•  Al nanorods synthesis •  Improved oxidation due to larger available surface area

•  Questions: 1.  What’s the size effect on the oxidation of Al-NRs? 2.  What’s the oxidation mechanism for Al-NRs?

Combustion Propagation for D = 36 nm Al-NR

core Al shell Al shell O environmental O

RDX

Single Molecule (CH2-N-NO2)3

1,3,5-Trinitroperhydro-1,3,5-triazine

1 unit cell a = 13.182, b = 11.574, c = 10.709Å

a = b = g = 90˚ Z=8 molecules (168 atoms) per unit cell

Space Group 61 Pbca

RDX: cyclotrimethylenetrinitramine “Research Department Explosive”

Applications: civil mining and military defense

Motivation: Shock-induced Detonation

•  PBXN 109 shock sensitivity test •  Ingredients: 64% RDX, 20% aluminum, etc.

Scientific questions: 1.  What is the reaction time? 2.  What is the reaction pathway (or intermediate

products)?

Detonation Speed

Reaction Time

Simulation reveals two-stage reactions in RDX detonation Large C-&O-rich clusters explain the slow release of CO

Fast reaction Slow reaction

Quantum Mechanical Validation QM (VASP)

QM confirms the stability of the large clusters at 1,300 K

ReaxFF MD

Thank You