Application of the ReaxFF reactive force fields to nanotechnology
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Application of the ReaxFF reactive force fields to nanotechnology. Adri van Duin, Weiqiao Deng, Hyon-Jee Lee, Kevin Nielson, Jonas Oxgaard and William Goddard III Materials and Process Simulation Center, California Institute of Technology. Contents.

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Application of the ReaxFF reactive force fields to nanotechnology

Adri van Duin, Weiqiao Deng, Hyon-Jee Lee, Kevin Nielson, Jonas Oxgaard and William Goddard III

Materials and Process Simulation Center, California Institute of Technology


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Contents nanotechnology

  • ReaxFF: background, rules and current development status

  • - Ni-catalyzed nanotube growth

  • - Validation of the all-carbon ReaxFF potential

  • - Building the Ni/NiC potential

  • - Testing the Ni-cluster description: magic number clusters

  • - Study of the initial stages of nanotube formation


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ReaxFF nanotechnology

Simulate bond formation

in larger molecular systems

ReaxFF: background and rules

Hierarchy of computational chemical methods

Empirical methods:

- Allow large systems

- Rigid connectivity

QC methods:

- Allow reactions

- Expensive, only

small systems

Atoms

Molecular

conformations

Design

years

Electrons

Bond formation

FEA

Time

MESO

Grids

MD

Grains

QC

Empirical

force fields

10-15

ab initio,

DFT,

HF

Ångstrom

Kilometres

Distance


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System energy description nanotechnology

2-body

3-body

4-body

multibody


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Key features nanotechnology

  • To get a smooth transition from nonbonded to single, double and triple bonded systems ReaxFF employs a bond length/bond order relationship. Bond orders are updated every iteration.

  • Nonbonded interactions (van der Waals, Coulomb) are calculated between every atom pair, irrespective of connectivity. Excessive close-range nonbonded interactions are avoided by shielding.

  • All connectivity-dependent interactions (i.e. valence and torsion angles) are made bond-order dependent, ensuring that their energy contributions disappear upon bond dissociation.

  • ReaxFF uses a geometry-dependent charge calculation scheme that accounts for polarization effects.


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General rules nanotechnology

- 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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Current status nanotechnology

  • ‘Finished’ ReaxFF force fields for:

  • Hydrocarbons (van Duin, Dasgupta, Lorant and Goddard, JPC-A 2001, 105, 9396)

  • (van Duin and Sinninghe Damste, Org. Geochem.2003, 34, 515

  • - Si/SiO2(van Duin, Strachan, Stewman, Zhang, Xu and Goddard, JPC-A 2003, 107, 3803)

  • Nitramines/RDX (Strachan, van Duin, Chakraborty, Dasupta and Goddard, PRL 2003,91,09301

  • Al/Al2O3(Zhang, Cagin, van Duin, Goddard, Qi and Hector, PRB in press)

  • Force fields in development for:

  • All-carbon materials

  • Transition metals, metal alloys and metals interacting with

  • first row elements

  • Proteins

  • Magnesium hydrides


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Ni-catalyzed nanotube growth nanotechnology

Concept: grow nanotubes from buckyball building blocks

Longer nanotube

- Exothermic reaction

- Huge activation barrier

- Probably needs catalyst


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Validation of the ReaxFF all-carbon potential nanotechnology

  • QC-data taken from hydrocarbon training set:

  • Single, double and triple bond dissociation

  • C-C-C, C-C-H and H-C-H angle bending

  • Rotational barriers around single, double and aromatic C-C bonds

  • Conformation energy differences

  • Methyl shift and H-shift barriers

  • Heats of formation for a large set of strained and unstrained non-conjugated, conjugated and radical hydrocarbons

  • Density and cohesive energies for diamond, graphite, cyclohexane and buckyball crystals

  • - All-carbon ReaxFF should also work for hydrocarbons


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All-carbon data added to the hydrocarbon training set nanotechnology

Relative energies for all-carbon phases

a: Experimental data; b: data generated using graphite force field (Guo et al. Nature 1991)

  • ReaxFF gives a good description of the relative stabilities of these structures


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  • Even-carbon acyclic compounds are more stable in the triplet state; odd-carbon, mono and polycyclic compounds are singlet states

  • Small acyclic rings have low symmetry ground states (both QC and ReaxFF)

  • ReaxFF reproduces the relative energies well for the larger (>C6) compounds; bigger deviations (but right trends) for smaller compounds

  • Also tested for the entire hydrocarbon training set; ReaxFF can describe both hydro- and all-carbon compounds


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Energy (kcal/mol) triplet state; odd-carbon, mono and polycyclic compounds are singlet states

Energy (kcal/mol)

C-C distance (Å)

  • ReaxFF gives good energies for key structures in buckyball growth

  • Training set includes all hydrocarbon cases used for ReaxFFCH


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Angle bending in C triplet state; odd-carbon, mono and polycyclic compounds are singlet states9

- ReaxFF properly describes angle bending, all the way towards the cyclization limit


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Diamond to graphite conversion triplet state; odd-carbon, mono and polycyclic compounds are singlet states

Calculated by expanding a 144 diamond supercell in the c-direction and relaxing

the a- and c axes

QC-data: barrier 0.165 eV/atom

(LDA-DFT, Fahy et al., PRB 1986, Vol. 34, 1191)

graphite

DE (eV/atom)

diamond

c-axis (Å)

  • ReaxFF gives a good description of the diamond-to-graphite reaction path


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Applications of all-carbon ReaxFF: buckyball+nanotube collisions

Impact velocity:

6 km/sec

(1500K)

Impact velocity:

9 km/sec

(2500K)


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Side impact collisions

  • Materials are too stable, extremely high impact velocities are required to start reaction

  • Catalyst required to lower reaction barriers


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Transition metal catalysis: Ni collisions

1: ReaxFF and QC EOS for Ni bulk phases

  • ReaxFF gives a good fit to the EOS of the stable phases (FCC, BCC, A15)

  • ReaxFF properly predicts the instability of the low-coordination phases (SC, Diamond)


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Testing the force fields for Ni magic number clusters collisions

Liquid

Energy/atom

(kcal)

Icosahedron

Amorphous

solid

FCC

MD-iterations


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MD-heatup/cooldown simulations collisions

Liquid

Energy/atom (kcal)

cooldown

heatup

  • ReaxFF gets the right trend for fcc/icosahedron transition

  • ReaxFF heat of melting converges on Ni bulk melting temperature (1720K)

Amorphous

solid

Temperature (K)

Icosahedron

FCC


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2. Results for Ni-C interactions collisions

Ni-C bond breaking in H3C-Ni-CH3

Energy (kcal/mol)

Ni-C bond breaking in Ni=CH2

Energy (kcal/mol)

Bond length (Å)


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Ni-C bond breaking in Ni(CH collisions3)4

Energy (kcal/mol)

Ni dissociation from 5-ring compound

Energy (kcal/mol)

Bond length (Å)


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Ni dissociation from 6-ring compound collisions

Energy (kcal/mol)

Ni dissociation from benzene

Energy (kcal/mol)

Bond length (Å)


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Ni dissociation from benzyne collisions

Energy (kcal/mol)

Bond length (Å)

C-Ni-C angle bending in benzyne/Ni complex

Energy (kcal/mol)

Angle (degrees)


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C-Ni-C angle bending in H collisions3C-Ni-CH3

Energy (kcal/mol)

Angle (degrees)


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Ni-assisted C collisions2-incorporation reactions

  • ReaxFFNi can describe the binding between Ni and C

  • A similar strategy has been used to make ReaxFF descriptions for Co/C and Cu/C, allowing us to compare their catalytic properties


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Influence adsorbed Ni on buckyball reactions collisions

ReaxFF-minimized buckyball

ReaxFF-minimized buckyball+2 Ni

2

2

1

1

R12= 1.45 Å

R12= 1.49 Å

  • ReaxFF predicts that buckyball C-C bonds get substantially weakened by adsorbed Ni-atoms

  • Might lower buckyball coalescence reaction activation barrier


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Energy (kcal/mol) collisions

Reaction coordinate

Influence adsorbed Ni on reaction barrier

Low-T ReaxFF restraint MD-simulation

  • Ni-atoms lower reaction barrier

  • Overall reaction becomes exothermic due to formation of Ni-Ni bonds

  • May explain Ni catalytic activity


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Influence Ni on initial stages of buckyball growth collisions

MD NVT-simulation (1500K); 5 C20-rings, 10 C4-chains (blank experiment)

t=125 ps.

t=0 ps.

  • C4 reacts with rings to form long acyclic chains

  • No branching


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MD NVT-simulation (1500K); 5 C collisions20-rings, 10 C4-chains and 15 Ni-atoms

t=125 to t=750 ps.

t=0 to t=125 ps.


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A closer look at the 750 ps. product collisions

  • Ni-atoms help create cage-structures

  • 750 ps. product has no internal C-C bonds

  • Ni-atoms leave ‘finished’ material alone and move away to defect and edge sites

  • Total simulation time: 4 days on 1 processor

  • Future work: Co, Fe


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Metal-catalyzed nanotube growth collisions

Inital configuration

  • Start configuration: 20 C6-rings, 5 metal atoms on edge

  • NVT simulation at 1500K

  • Add C2-molecule every 100,000 iterations


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Metal-catalyzed nanotube growth collisions

Results after 2,000,000 iterations

Metal=Ni

Metal=Co

No metal

Metal=Cu

-Ni and Co lead to greatly enhanced ring formation. Cu is far less active.


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Conclusions collisions

  • ReaxFF has proven to be transferable to transition metals and can handle both complex chemistry and chemical diversity

  • The low computational cost of ReaxFF (compared to QC) makes the method highly suitable for screening heterogeneous and homogeneous transition metal catalysts


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