Reactive Force Field & Molecular Dynamics Simulations (Theory & Applications)

Transcription

1 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 Feb 19 th, 2013

2 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

3 Research Background Objective: Understanding the combustion mechanism of energetic materials by atomistic simulations Challenge: Reactive atomistic simulations Large scale (multimillion atoms) Long time (nanosecond)

4 Simulation Method Reactive MD Reactive force field (RMD & ReaxFF) Al Nanostructures RDX TATB

5 Reactive Force Fields (RMD) Ceramic Al 2 O 3 (Vashishta et al., 2008) Al EAM Potential (Voter and Chen, 1987) Hybrid Potential for Al-O system

6 Reactive Force Fields (RMD) Ceramic Al 2 O 3 (Vashishta et al., 2008) Metallic Al (Voter and Chen, 1987)

7 Environmental Dependent Interpolation Coupling of two potentials Oxidation degree:

8 Validation of Al 2 O 3 Potential Cohesive energy, elastic constants and melting Liquid structure P. Vashishta et al., J. Appl. Phys. 103, (2008)

9 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)

10 Validation of RMD for Al x O y Comparison of bond length and bond angle in Al x O y fragments QM W. Wang, PhD Thesis, USC (2008) P. Politzer et al., J. Phys. Chem. A, 105, (2001)

11 Reactive Force Fields (ReaxFF)

12 Classification of ReaxFF Potential E = E lp + E over + E under + E bond + E val + E pen + E coa Bonded + E tors + E conj + E hbond + E vdwaals + E coulomb Non-bonded Coordination, 2-body, 3-body, 4-body, hydrogen bonding, and nonbonding energies

13 Bond Order (ReaxFF) Reactive bond order : {BO ij } Bond breakage & formation Bond Order

14 Bond Energy (ReaxFF: Term 4) Reactive bond order potential energy: E bond ({r ij },{BO ij }) Bond breakage & formation Bond Energy Developed by Goddard, van Duin, et al. (Caltech)

15 Variable Charge Problem (ReaxFF: Term12) Charge-equilibration (QEq) Charge transfer Determine atomic charges {q i i = 1,..., N} every MD step to minimize E ES (r N,q N ) with charge-neutrality constraint: Σ i q i = 0 O 2 dissociation on Al(111)

16 Published ReaxFF Force ReaxFF: Fields: Periodic Table - Hydrocarbons (van Duin, Dasgupta, Lorant & Goddard, JPC-A 01, 105, 9396; van Duin & Sinninghe Damste, Org. Geochem. 03, 34, 515) - Si/SiO 2 (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, ; Han, Strachan, van Duin & Goddard, in preparation; van Duin, Dubnikova, Zeiri, Kosloff & Goddard, submitted to JACS) - Al/Al 2 O 3 (Zhang, Cagin, van Duin, Goddard, Qi & Hector, PRB 04, 69, ) - 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

17 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, (2003)

18 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

19 Linked List Cell Normal Traversal: Search space: Full system Computation: O(N 2 ) Linked List Cell Length of each cell is at least equal to r cut Search space: Only neighboring cells Computation: O(N)

20 Scalability of Parallel ReaxFF MD N Weak scaling N = P P Parallel efficiency is on 786,432 BG/Q cores for ReaxFF on 8,455,716,864 atoms RDX

21 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

22 Aluminum Nanostructures Experimental Background Aluminum combustion reaction Aluminum nanoparticle (Al-NP) microscale vs. nanoscale core metallic Al shell 2~4 nm amorphous Al2O3 Y. Sun et al., Defense Science J., 4, 56 (2006) Y. A. Kotov et al., Nanotech. in Russia, 4 (2009) Close packed Al-NP aggregate Vapor deposited Al-nanorod Y. Gan et al., Combust. Flame, 158, (2011) C. Li, et al., Chem. Mater. 19, (2007)

23 Oxidation of Al-NP Aggregates In air Small fragments With CuO Large agglomerations Y. Ohkura, et al., Combust. Flame, 158, (2011) 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)?

24 Combustion Propagation for D = 26 nm Al-NP Aggregate core Al shell Al shell O

25 Al Nanorods Oxidation Al nanorods synthesis Improved oxidation due to larger available surface area S. K. Cheah et al., Nano Letters 10, 9, (2009) Questions: 1. What s the size effect on the oxidation of Al-NRs? 2. What s the oxidation mechanism for Al-NRs?

26 Combustion Propagation for D = 36 nm Al-NR core Al shell Al shell O environmental O

27 RDX RDX: cyclotrimethylenetrinitramine Research Department Explosive Applications: civil mining and military defense Single Molecule (CH 2 -N-NO 2 ) 3 1,3,5-Trinitroperhydro-1,3,5-triazine 1 unit cell a = , b = , c = Å a = b = g = 90 Z=8 molecules (168 atoms) per unit cell Space Group 61 Pbca

28 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)?

29 Detonation Speed

30 Reaction Time Fast reaction Slow reaction Simulation reveals two-stage reactions in RDX detonation Large C-&O-rich clusters explain the slow release of CO

31 Quantum Mechanical Validation ReaxFF MD QM (VASP) QM confirms the stability of the large clusters at 1,300 K

32 Thank You