development and applications of reaxff reactive force ......development and applications of reaxff...
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Adri van Duin
Associate Professor Department of Mechanical and Nuclear Engineering
Pennsylvania State University, 136 Research East Building phone: 814-8636277; E-mail [email protected]
Development and applications of ReaxFF reactive force fields for combustion, catalysis and material failure
NIST 2010 Workshop on Atomistic Simulations for Industrial Needs, July 27
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Solving the size/time gap between simulation and experiment
Tim
e
Distance Ångstrom Meters
10-15
Hours
Atomistic simulations
Experiment Improving experimental resolution
Increasing computer
speed
QM
Atomistic simulations
Increasing computer
speed
ReaxFF
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Outline
- Simulations on the dynamics of chemical reactions - The ReaxFF reactive force field - Overview of ReaxFF applications
- Combustion reactions - Material failure - Catalysis
- Summary
ReaxFF MD-simulation on soot combustion at elevated oxygen pressure. Soot structure provided
by Jonathan Matthews (Penn State)
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Force field methods
©2005 Markus Buehler, MIT
b
φ
Θ
- Empirical, we need to derive values for the force field parameters (intuition, compare to experiment, compare to QM) - MUCH faster than QM; can be applied to bigger systems
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C-C bond length (Å)
Ene
rgy
(kca
l/mol
)
C-C bond stretching in Ethane Around the equilibrium bond length Full dissociation curve
C-C bond length (Å)
- Although the harmonic approximation can describe the bond stretching around the equilibrium it cannot describe the bond dissociation.
Failure of the harmonic model
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- To get a smooth transition from nonbonded to single, double and triple bonded systems ReaxFF employs a bond length/bond order relationship [1-3]. Bond orders are updated every iteration.
- All connectivity-dependent interactions (i.e. valence and torsion angles) are made bond-order dependent, ensuring that their energy contributions disappear upon bond dissociation [4].
- Nonbonded interactions (van der Waals, Coulomb) are calculated between every atom pair, irrespective of connectivity. Excessive close-range nonbonded interactions are avoided by shielding [4].
- ReaxFF uses a geometry-dependent charge calculation scheme [5] that accounts for polarization effects.
From non-reactive to reactive force fields: key features of ReaxFF
1. Abell, G. C., (1985) Phys. Rev. B 31. 2. Brenner, D. W., (1990) Phys. Rev. B 42, 9458-9471 3. Tersoff, J., (1988) Phys. Rev. Lett. 61, 2879-2882. 4. van Duin, A. C. T., Dasgupta, S., Lorant, F., and Goddard, W. A., (2001) J. Phys. Chem. A 105, 9396-9409. 5. Mortier, W. J., Ghosh, S. K., and Shankar, S. (1986) JACS 108, 4315-4320.
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Calculation of bond orders from interatomic distances
Introduction of bond orders
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General rules for ReaxFF
- MD-force field; no discontinuities in energy or forces even during reactions.
- User should not have to pre-define reactive sites or reaction pathways; potential functions should be able to automatically handle coordination changes associated with reactions.
- Each element is represented by only 1 atom type in the force field; force field should be able to determine equilibrium bond lengths, valence angles etc. from chemical environment.
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0.01
0.1
1
10
100
1000
10000
100000
1000000
0 100 200 300 400
ReaxFF
QM (DFT)
Nr. of atoms
Tim
e/ite
ratio
n (s
econ
ds)
ReaxFF Computational expense
x 1000,000 - ReaxFF allows for reactive MD-simulations on systems containing more than 1000 atoms - ReaxFF is 10-50 times slower than non-reactive force fields - Better scaling than QM-methods (NlogN for ReaxFF, N3 (at best) for QM
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- ReaxFF combines covalent, metallic and ionic elements allowing applications all across the periodic table - All ReaxFF descriptions use the same potential functions, enabling application to interfaces between different material types - Over 60 publications in open literature - Code has been distributed to over 125 research groups - C/H/O potential available on NIST websites - Parallel ReaxFF (LAMMPS/Reax, open source) available - Incorporated in ADF/BAND graphical user interface
not currently described by ReaxFF
Current development status of ReaxFF
ReaxFF transferability
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ReaxFF integration into ADF/BAND graphical user interface - Collaboration with SCM. - Professional software support team. - Beta-version GUI available - Parallel version + code optimization planned. - Direct integration of state-of-the art DFT/FF/TB codes.
Integration team: - Stan van Gisbergen, Olivier Visser, Alexei Yakovlev (SCM) - Mike Russo, Kaushik Joshi (Penn State)
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Combustion reactions
QM
ReaxFF
Engineering
- Accurate energies for key reaction paths - Size range: up to around 100 atoms - Mostly static, highly limited dynamics
- Allows nanosecond-dynamics for systems up to 107 atoms - Can be parameterized directly from QM
Database of reaction kinetics and diffusion constants
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QM ReaxFF
Force field development: hydrocarbon oxidation
Oxidation reactions
Rotational barriers Angle strain
- total training set contains about 1700 compounds
QM: Jaguar/DFT/B3LYP/6-311G**
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Test ReaxFF CHO-description: oxidation of o-xylene
2 o-Xylene; 70 O2 in 20x20x20 Angstrom box ReaxFF NVT/MD at T=2500K
- Exothermic reaction - Exothermic events are related to H2O and CO2 formation
- Oxidation initiates with OOH-formation - Final products dominated by CO, CO2 and H2O
Consumed O2
CO2
H2O
CO o-Xylene
OOH OH
Chenoweth, van Duin and Goddard, JPC-A 2008
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o-Xylene oxidation: Detailed reaction mechanism
- Reaction initiation with HO2-formation
- Dehydrogenation occurs at methyl-groups, not at benzyl-hydrogens
- Only after H2C=O is formed and dissociated the benzene ring gets oxidized
- Ring opens shortly after destruction of aromatic system
- Ring-opened structure reacts quickly with oxygen, forming CO2, H2O and CO
- ReaxFF gives sensible predictions for the o-Xylene oxidation mechanism. - This mechanism can be further validated against QM
C H 3 C H 3 C H 3
O 2 C H 2 C H 3 H O 2
H 2 C O O H frame 128 frame 174
frame 175 H 2 C O C H 3 O H
frame 176 C H 3 H 2 C = O
O H C H 3 O O
H 2 C = O O H
frame 176 O H
C H 2 O
H 2 C = O O H O H C H 2
O H C = O
O H C H 2 O
O H O O H O
C H 2 O
O H C O 2 H O 2 H O 2
frame 177
frame 179 frame 180 frame 182
frame 193
O 2
O 2
O H 2
O H 2 O H 2
O H C O 2 H H H H
H H O
C O 2 O H
H H H H
H H C O
C O 2 O
H H H H
H H C O O 2 H
C O 2 O
H H H
H H H C O O 2 H
O H
H H H H H
O O C O 2
C O O 2 H
H H H O O H H
H C = O C O 2
O H 2 C O
O 2 H
frame 205
frame 209
frame 232 frame 232 frame 234
O 2 O 2
O H 2
O H 2 O H 2 O H 2 O H 2 O H 2
O H 2
O H 2 O H 2 O H 2 O H 2
Radical positions not marked after frame 193
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Exo-tetrahydrodicyclopentadiene: JP-10
• Synthetic high-energy density fuel – Volumetric heat content: 39434 MJ/m3 (141500 BTU/gal); high strain
leads to high energy output – Potential use in pulse detonation engines and missile applications – Single component fuel
• Mechanism is incomplete – No clean mechanism for either pyrolysis and combustion – Early stages of fragmentation and oxidation are important for
combustion models • Goal
– Use ReaxFF to predict initiation mechanisms and kinetics – Validate predictions with QM and experiment
Initiation Mechanism and Kinetics for Pyrolysis and Combustion of JP-10
with Kimberly Chenoweth, Siddharth Dasgupta and Bill Goddard (Caltech)
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Simulation strategy for JP-10 pyrolysis
• 40 JP-10 Molecules (1040 atoms) • Density = 0.388 g/cm3 • Equilibrated at 1500K for 10 ps • Temperature control using Berendsen thermostat with 0.1 ps damping constant • MD time step of 0.1 fs • Total simulation time: 50 ps • Perform a series of NVT-MD simulations to calculate activation energy and product distribution
• Temperature (K): 2000, 2100, 2200, 2300, 2400, 2500, 2600 • 10 simulations at each temperature with unique starting configuration • Results obtained by averaging over the 10 simulations at each temperature
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Kinetics for JP-10 Pyrolysis
- Excellent agreement with experiment (Rao and Kunzru, J. Anal. Appl. Pyrolysis 2006)
Ea (ReaxFF) = 62.09 kcal/mol Ea (Expt) = 62.48 kcal/mol
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Simulation strategy for JP-10 oxidation
• 5 JP-10 molecules • 10 oxygen radicals • 140 atoms • Density = 0.174 g/cm3 • Equilibrated at 1000K for 5 ps • Temperature control using Berendsen thermostat with 0.1 ps damping constant • MD time step of 0.1 fs • Total simulation time: 10 ps • Performed a NVT-MD simulation at 1000K • Perform QM calculations on intermediates to validate ReaxFF results
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JP-10 oxidation
• Reaction mechanism observed during NVT-MD simulation at 1000K • Overall energetics in good agreement with QM • By fitting ReaxFF against a large database of QM-data we obtain a method that is transferable to new systems • JP-10 QM/ReaxFF comparison can be fed back into training set allowing the force field to further improve • Full control of pressure and temperature
ReaxFF
QM (B3LYP/6-311G**)
Chenoweth, van Duin, Dasgupta and Goddard, J.Phys.Chem.A 2009
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LAMMPS/ReaxFF Parallel MD-simulation on hydrocarbon combustion T=2500K, n=12,000 atoms, 4 processors; CPU-time: 4 hours
Simulation by Mike Russo, Penn State
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Initial application to coal pyrolysis (ADF/ReaxFF)
MD simulation with temperature ramp
Coal structure, 20,000 atoms C:S ratio: 100:1, from Fringe3D [1]
[1] Ferdandez-Alos, V., J.K. Watson, and J.P. Mathews. Directly capturing aromatic structural features in coal via "Fringe3D" generating 3D molecular models directly from HRTEM lattice fringe images. in Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2009. Salt Lake City, UT.
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Material failure QM
ReaxFF
Finite element
H-pathway from 3-fold [110] to subsurface tetrahedral site
Impurity migration in Fe-polycrystallite
H-diffusion at T=500K on Fe[110] surface
- Accurate energies for key reaction paths - Size range: up to around 100 atoms - Mostly static, highly limited dynamics
- Allows nanosecond-dynamics for systems up to 107 atoms - Can be parameterized directly from QM; no electronic info
- Size/time ranges suitable for engineering applications
Database of temperature, pressure and concentration dependent diffusion/decohesion rules
Fe
H
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Development of a ReaxFF potential for Ni/S systems
- Other data in training set: cohesive energy, surface energy (111 and 100), vacancy formation energy
Ni-bulk metal
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Sulfur binding energies to Ni-surface and subsurface sites
Ni7Vac1 Ni7S1
ΔEQM=95.0 kcal/molS ΔEReax=94.9 kcal/molS
- Sulfur strongly prefers surface-sites
QM-data from Mu-Jeng Cheng, Yao Sha and Yi Liu (MSC), using SeqQuest (Peter Schultz, SNL)
Ni
S
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Simulations on material failure as a function of S-concentration
- Ni7S-phase; expansion in 111-direction - 2048 atoms; T=300K; NVT-simulation
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Simulations on material failure as a function of S-concentration
- Sulfur significantly weakens the material - Surface formation becomes exothermic at high sulfur concentrations
Chen, Kalla, Kaxiras, Lu, Nakano, Nomura, van Duin, Vashista and Yuan, PRL (2010)
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Simulation of ZnO nanowire response to local heating With Mike Russo and Arvind Bharati
Central region: temperature pulse of 2500K Rest of the structure: NVE-dynamics
Collaboration with David Raymand and Kersti Hermannsson (Uppsala) [1] Raymand, van Duin, Baudin and Hermannsson, Surface Science 2008 [2] Raymand, van Duin, Spangberg, Hermannson and Goddard, Surface Science 2010
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Analysis of ZnO nanowire surface chemistry
- Rapid adsorption, followed by dissociation, of water molecules - Dissociated water assists in the breakup of the nanowire - Release of O2 molecules from the ZnO-nanowire surface - ReaxFF can be used to test the high-temperature stability of metal oxide nanowire systems in water vapor - Extension to Zn/N and Zn/C chemistry (under development) will allow simulations in a combustion environment
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Catalysis
QM
ReaxFF
Engineering
- Accurate energies for key reaction paths - Size range: up to around 100 atoms
- Allows nanosecond-dynamics on multi-component systems - Can be parameterized directly from QM
- Interface properties - Reaction kinetics - Diffusion constants
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QM-data (Greeley and Mavrikakis, 2004)
CH3OH(g)
CH3OH (ads) H+CH2OH 2H+CHOH
3H+COH 4H+CO
2H2(g)+CO(g)
2H+CH2O H+CH3O
TS TS
TS TS
TS
- Excellent agreement between ReaxFF and QM for entire reaction path
QM/ReaxFF methanol reaction pathways
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MD-simulation at T=1250K on methanol/Pt[111]
- Methanol dissociates on Pt-surface, generating adsorbed hydrogen - CO accumulates on surface (poisoning) - ReaxFF descriptions for Ti, Mo, V, Bi, Ru, Ni available
H(ads)
Methanol(g)
CO(ads) H2COH(ads) HCOH(ads)
COH(ads)
Time(ps)
Com
poun
d fre
quen
cy
Methanol conversion on Pt[111]-surface
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- ReaxFF has proven to be transferable to a wide range of materials and can handle both complex chemistry and chemical diversity. Specifically, ReaxFF can describe covalent, metallic and ionic materials and interactions between these material types.
- The low computational cost of ReaxFF (compared to QM) makes the method suitable for simulating reaction dynamics for large (>> 1000 atoms) systems (single processor). ReaxFF has now been parallelized, allowing reactive simulations on >>1000,000 atoms.
: not currently described by ReaxFF
Summary
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Acknowledgements / Contact information
Website: http://www.engr.psu.edu/adri/
Phone: 814-863-6277 E-mail: [email protected]
ReaxFF simulation of the collision between two
closed carbon nanotubes filled with highly-reactive N20-molecules. Impact
velocity: 4 km/sec
Collaborators: - Satyam Agrawalla, Amar Kamat and Mike Russo (Penn State) - Jonathan Mathews, Jim Kubicki, Mike Janik (Penn State) - David Raymand, Kersti Hermannson (Uppsala) - Kimberley Chenoweth, Vyacheslav Bryantsev, Bill Goddard (Caltech) - Obaidue Rahaman (Ramie) and Doug Doren (U. Delaware)
Funding: - PSU/KISK startup grant - Illinois Coal - GM and Exxon (ADF integration) - British Royal Society (initial ReaxFF funding)