ReaxFF

Key takeaways

ReaxFF belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect ReaxFF to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of ReaxFF from memory before moving on to harder problems.

Reference excerpt

ReaxFF (for "reactive force field") is a bond order-based force field developed by Adri van Duin, William A. Goddard, III, and co-workers at the California Institute of Technology. One of its applications is molecular dynamics simulations. Whereas traditional force fields are unable to model chemical reactions because of the requirement of breaking and forming bonds (a force field's functional form depends on having all bonds defined explicitly), ReaxFF eschews explicit bonds in favor of bond orders, which allows for continuous bond formation/breaking. ReaxFF aims to be as general as possible and has been parameterized and tested for hydrocarbon reactions, alkoxysilane gelation, transition-metal-catalyzed nanotube formation, and many advanced material applications such as Li ion batteries, TiO2, polymers, and high-energy materials. To be able to deal with bond breaking and formation whilst having only 1 single atom type for each element, ReaxFF is a fairly complex force field with many parameters. Therefore an extensive training set is necessary covering the relevant chemical phase space, including bond and angle stretches, activation and reaction energies, equation of state, surface energies, and much more. Usually, but not necessarily, the training data is generated with electronic structure methods. In practice, often DFT calculations are used as a pragmatic approach, especially since more accurate functionals are available. For the parameterization of such a complex force field, global optimization techniques offer the best chance to get a parameter set that most closely describes the training data.

References

van Duin, Adri C. T.; Dasgupta, Siddharth; Lorant, Francois; Goddard, William A. (2001). "ReaxFF: A Reactive Force Field for Hydrocarbons" (PDF). The Journal of Physical Chemistry A. 105 (41): 9396–9409. Bibcode:2001JPCA..105.9396V. doi:10.1021/jp004368u. Nielson, Kevin D.; van Duin, Adri C. T.; Oxgaard, Jonas; Deng, Wei-Qiao; Goddard, William A. (2005). "Development of the ReaxFF Reactive Force Field for Describing Transition Metal Catalyzed Reactions, with Application to the Initial Stages of the Catalytic Formation of Carbon Nanotubes" (PDF). The Journal of Physical Chemistry A. 109 (3): 493–499. Bibcode:2005JPCA..109..493N. doi:10.1021/jp046244d. PMID 16833370. Buehler, M.; Van Duin, A.; Goddard, W. A. (2006). "Multiparadigm Modeling of Dynamical Crack Propagation in Silicon Using a Reactive Force Field" (PDF). Physical Review Letters. 96 (9) 095505. Bibcode:2006PhRvL..96i5505B. doi:10.1103/PhysRevLett.96.095505. PMID 16606278. Strachan, A.; Kober, E. M.; Van Duin, A. C. T.; Oxgaard, J.; Goddard, W. A. (2005). "Thermal decomposition of RDX from reactive molecular dynamics" (PDF). The Journal of Chemical Physics. 122 (5): 054502. Bibcode:2005JChPh.122e4502S. doi:10.1063/1.1831277. PMID 15740334. Strachan, A.; Van Duin, A. C. T.; Chakraborty, D.; Dasgupta, S.; Goddard, W. A. (2003). "Shock Waves in High-Energy Materials: The Initial Chemical Events in Nitramine RDX" (PDF). Physical Review Letters. 91 (9) 098301. Bibcode:2003PhRvL..91i8301S. doi:10.1103/PhysRevLett.91.098301. PMID 14525217. Buehler, M.; Tang, H.; Van Duin, A. C. T.; Goddard, W. A. (2007). "Threshold Crack Speed Controls Dynamical Fracture of Silicon Single Crystals" (PDF). Physical Review Letters. 99 (16) 165502. Bibcode:2007PhRvL..99p5502B. doi:10.1103/PhysRevLett.99.165502. PMID 17995264. Ojwang, J. G. O.; Van Santen, R.; Kramer, G. J.; Van Duin, A. C. T.; Goddard, W. A. (2008). "Modeling the sorption dynamics of NaH using a reactive force field" (PDF). The Journal of Chemical Physics. 128 (16): 164714. Bibcode:2008JChPh.128p4714O. doi:10.1063/1.2908737. PMID 18447486. Kulkarni, A. D.; Truhlar, D. G.; Goverapet Srinivasan, S.; Van Duin, A. C. T.; Norman, P.; Schwartzentruber, T. E. (2013). "Oxygen Interactions with Silica Surfaces: Coupled Cluster and Density Functional Investigation and the Development of a New ReaxFF Potential". The Journal of Physical Chemistry C. 117: 258–269. doi:10.1021/jp3086649. Deetz, J. D.; Faller, R. (2014). "Parallel Optimization of a Reactive Force Field for Polycondensation of Alkoxysilanes". The Journal of Physical Chemistry B. 118 (37): 10966–10978. doi:10.1021/jp504138r. PMID 25153668.

External links Adri van Duin's Website ReaxFF in LAMMPS ReaxFF in the Amsterdam Modeling Suite ReaxFF in PuReMD (Purdue Reactive MD) suite

Worked examples

Example 1 — a first encounter with ReaxFF

Start with the simplest possible case. Write down what ReaxFF claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to ReaxFF before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about ReaxFF ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of ReaxFF

In research: ReaxFF appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses ReaxFF in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: ReaxFF is common in secondary-school and first-year university syllabi. It links to neighbouring topics Force fields (chemistry), so understanding it makes those chapters shorter.
In everyday life: Look for ReaxFF outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study ReaxFF in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what ReaxFF means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain ReaxFF out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is ReaxFF in simple terms?

ReaxFF (for "reactive force field") is a bond order-based force field developed by Adri van Duin, William A. Goddard, III, and co-workers at the California Institute of Technology.

Why does ReaxFF matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study ReaxFF?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on ReaxFF.