Force ReaxFF

Goddard et al. developed and validated the reactive force field (ReaxFF) to describe complex reactions (including catalysis) nearly as accurately as QM in some cases but at computational effort comparable to classical molecular dynamics (MD).10,18 Similar to empirical non-reactive force fields, the reactive force field divides the system energy up into various partial energy contributions,... 

ReaxFF describes the total energy of an atomistic system with three main terms i) covalent (bonds, angles, torsions, etc.), ii) electrostatics with environment-dependent charges, and iii) van der Waals interactions. Covalent interactions are based on the concept of partial bond orders that are calculated solely from atomic positions (no pre-determined connectivities). Once the bond order between every pair of atoms is known, bond energies, angles, and torsions are determined. The second key concept in reactive force fields (also used in the... 

In addition to the classical force fields above, many other force fields have been developed for small drug molecules or macromolecules. The MM2, MM3, and MM4 force fields were developed by Norman L. Allinger for a broad range of chemicals, and CFF is a family of force fields adapted to a broad variety of organic compounds, polymers, metals, and so on. The MMFF force field was developed at Merck for a broad range of chemicals. ReaxFF is a reactive force field, developed by William Goddard and coworkers, is fast, transferable, and the computational method of choice for atomistic-scale dynamics simulations of chemical reactions. 

The results presented in Fig. 4 show the evolution of the number of adsorbed benzene in the different system as a function of the simulation time. The ReaxFF force field allows the creation and breaking of covalent bonds between the different atoms of the system during the molecular dynamics simulation. In fliis work, we considered that a benzene molecule was adsorbed, if it formed at least one bond with the Ni (100), Ni (111), or Raney-Nickel surface, respectively. In the first 5 ps, the benzene adsorption is comparable for all three systems evaluated, i.e. both clean Ni surfaces and Raney Ni model. After the first 5 ps, about 6-7 % of benzene molecules have been adsorbed. In the very beginning of the simulation time, the adsorption process is even faster on Ni (100) and Ni (111) surface (blue and green line in Fig. 4) compared to the catalyst (purple line). After the first few ps, the adsorption on Ni (100) surface (blue line in Fig. 4) remains rather constant and does not increase much. After 25 ps, only about 12 % of benzene have been adsorbed. In contrast to this finding, the benzene adsorption on the Ni (111) surface and the Raney Ni model system (green and purple line in Fig. 4) increases more... 

Mueller, J.E., van Duin, A.C.T., Goddard, W.A. Application of the ReaxFF reactive force field to reactive d5mamics of hydrocarbon chemisorption and decomposition. J. Phys. Chem. C 114, 5675-5685 (2010)... 

Besides electrode surface structure, the nature of the electrode and solvent also affect the evolution of the SEI. An interesting and detailed simulation of formation and growth of SEI on Li metal surface in EC, DMC, and EC mixed with DMC electrolyte was done by Kim et al., using reactive force field (ReaxFF) molecular simulations [61]. The SEI film was found to grow faster in EC-based electrolyte compared to DMC, generating thicker SEI film, and EC mixed with DMC electrolyte came in between, as shown in Fig. 5.18. This simulation result agrees with the... 

In addition to QC studies, reactive molecular dynamics (RMD) simulations using the reactive force field ReaxFF have been used to gain insight into reactions of singly reduced EC in the condensed (solution) phase [31]. In this study the reaction of Li /o-EC with both LiVo-EC and LiVc-EC has been studied in a solution of EC molecules. A snapshot of the system is shown in Fig. 7.5. RMD simulations were used to determine the free energy as a function of reaction coordinate (see below) and to examine the propensity of various radical combination reactions to occur in the condensed phase of an EC solvent. 

The ReaxFF Force Field for Studying Reactive Processes... 

ReaxFF [50] provides a generally valid and accurate way to capture the barriers for various chemical reaction processes (allowed and forbidden reactions) into the force fields needed for large-scale MD simulation. ReaxFF is parameterized exclusively from QM calculations, and has been shown to reproduce the energy surfaces, structures, and reaction barriers for reactive systems at nearly the accuracy of QM but at costs nearly as low as conventional FFs. 

While the vast majority of theoretical studies of CNT growth starts with lone carbon atoms, assuming that decomposition has already taken place, there are conditions (e.g., low temperature growth) under which decomposition is believed to be the rate-limiting step [87]. Thus we have utilized this ReaxFF force field in a reactive dynamics (RD) study of the early stages of CNT growth. In [85] we reported on the chemisorption and decomposition of various hydrocarbon species on a nickel nanoparticle. Over the course of 100 ps of RD simulations performed, we were able map out the preferred reaction pathways for the decomposition of each hydrocarbon species studied. 

As effective as reaxFF is for handling reactive systems and processes in their ground-state, it is unable to describe the dynamics of electrons and systems with excited electroific states. QM-MD is also limited mostly to ground-state dynamics or to a very small number of excited electronic states (see [90] for further discussion on this). The following section presents our progress in addressing this problem with a mixed quantum-classical force field method, the eFF. 

Cheung S et al (2005) ReaxFF(MgH) reactive force field for magnesium hydride systeans. J Phys Chem A 109(5) 851-859... 

Ludwig J et al (2006) Dynamics of the dissociation of hydrogen on stepped platinum surfaces using the ReaxFF reactive force field. J Phys Chem B 110(9) 4274—4282... 

Mueller JE, van Duin ACT, Goddard WA (2010) Development and validation of ReaxFF reactive force field for hydrocarbon chemistry catalyzed by nickel. J Phys Chem C 114(11) ... 

Nielson KD, van Duin ACT, Oxgaard J, Deng W-Q, Goddard WA II (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. J Phys Chem A 109 493 99... 

Chenoweth K, van Duin ACT, Persson P, Cheng M-J, Oxgaard J, Goddard WA III (2008) Development and application of a ReaxFF reactive force field for oxidative dehydrogenation on vanadium oxide catalysts. J Phys Chem C 112 14645-14654... 

Van Duin, A.C.T., Merinov, B.V., Jang, S.S., Goddard 111, W.A. (2008) ReaxFF reactive force field for solid oxide fuel cell systems with application to oxygen ion transport in yttria-stabilized zirconia. J. Phys. Chem. A, 112 (14), 3133-3140. Mazumder, S.K., Pradhan, S.K., Hartvigsen, J., Rancmel, D., von Spakovsky, M.R., and Khaleel,... 

ReaxFF reactive force field for solid oxide fuel cell systems with application to oxygen ion transport in yttria-stabilized zirconia. J. Phys. Chem. A, 112 (14), 3133-3140. 

Reactive molecular dynamics (ReaxMD) is based on the Reax force field (ReaxFF) parameterized by fitting to a training set of QM data. Compared to first-principles simulations, with the ReaxFF model it is possible to speed up the calculation by several orders of magnitude. However, due to the enormous complexity of the underlying mathematical expressions, ReaxFF is -10 to 100 times more expensive computationally than simple (nonreactive) FFs. 

Other popular atomic reactive FFs include the reactive molecular dynamics (RMD) modef and the Reax force field (ReaxFF). The potential importance of ReaxFF in advancing realistic MD simulations merits a short discussion. 

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