Reactive force-field

MM, UFF, molecular mechanics, universal force field. Ref. (14). Charge transfer reactive force field. Ref. (92). 

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,... 

Describe the main approaches to the construction of empirical force fields for molecular dynamic simulations. Describe the difference between ordinary and reactive force fields. 

Another reactive force field that is dependent on bond-order was developed by van Duin, Dasgupta, Loran, and Goddard [183] for hydrocarbons. The configurational energy is described as the sum of energy contributions from internal modes as well as non-bonding van der Waals and Coulombic interactions, but the parameters of the functions that describe each contribution is dependent upon the bond order of atoms involved in each description. It is assumed that the bond order between an atom pair is dependent on the interatomic separation. While this model has been used to predict bond dissociation energies, heats of formation and structures of simple hydrocarbons, it was not applied to predict condensed phase properties. However, the form of the potential should allow for condensed phase studies. 

Progress was also reported in modeling the reaction and transportation processes on fuel cell catalysts and through membranes, using multiple paradigms as well as starting from first principle quantum mechanics to train a reactive force field that can be applied for large scale molecular dynamics simulations. It is expected that the model would enable the conception, synthesis, fabrication, characterization, and development of advanced materials and structures for fuel cells . 

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... 

Once the bond orders have been calculated, they are used to compute bond energies, angles, and torsions. These terms are also used in non-reactive force fields, but their use in reactive simulations requires some modifications. All covalent terms are pre-multiplied by the bond orders involved this ensures that whenever a bond is broken, all the terms involving it vanish smoothlv. Also, the equilibrium angle in covalent-angle terms depends upon the bond orders... 

Despite this progress several areas require further development. For example, we have evidenced that only a limited number of force fields are presently available for treatment of ionic salts, It will be very beneficial that this gap will be filled and general, transferable sets of force fields for different classes of ionic systems will be available as is the case with other classes of energetic materials such as nitramines systems. We have also pointed out in this chapter that current classical force fields developed for ionic crystals are limited to description of nonreactive processes. Development of reactive force fields such as reactive empirical bond order potentials for the case of ionic systems will represent a major forward step for simulation of reactions and of combustion and denotation processes. 

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. 

Other approaches to model transition states with force fields are the empirical valence bond model (EVB). [336] the reactive force field (RFF) [337] and the multiconfigurational molecular mechanics (MCMM) method [338]. 

Lammers S, Lutz S, Meuwly M (2008) Reactive force fields for proton fransfer dynamics. J Comput Chem 29 1048... 

Reactive Force Field Simulations on Benzene Adsorption on the Raney-Nickel Catalyst... 

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)... 

The AIMD method, based on the Carr and ParrineUo approach [127], has also been applied in the study of electrochemistry [128]. Reactive Force Field approaches are now being used to study the ionomer/water/catalyst interfaces during an electrochemical reaction [129]. Neurock et al. developed a detailed first-principles approach that employs a double-reference method to simulate the influence of the electrochemical potential on the chemistry at the metal/solution interface [130]. hi this method the aqueous solution metal interface and the interfacial potential drop are explicitly treated. However the choice of an appropriate water surface structure is critical for establishing the appropriate electrochemical behavior at the atomistic scale. This method has been applied to smdy some electrochemical steps involved in the ORR and methanol oxidation on Pt (e.g. [131, 132]). 

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... 

Also Bedrov et al. have recently addressed EC with a combination of static and dynamic approaches high-level DFT with classic MD using a reactive force field [32]. Kim et al., with a similar MD approach, deflned an SEI formation potential with predicted values of 0.9,1.1, and 1.0 V vs. Li/LP for DMC, EC, and EC+DMC electrolytes in contact with a lithium metal surface [47]. 

By using molecular dynamics with a reactive force field [28-30] as implemented in LAMMPS [31], we were able to analyze the side effects of ion bombardment on a sihca-supported single waUed carbon nanotube. A reactive force field enables simulating the breaking and formation of covalent bonds. Apart from observing the effective removal of carbon atoms, we found the possibility of undesired effects on the carbon nanotube sidewall, on the substrate as well as at the interface between the carbon nanotube and the substrate (Fig. 7.1). We highlight the main types of atomic defect found on carbon nanotube sidewall, vacancy defects and chemisorption. 

A limitation of many classes of interatomic potential is that the identity of an atom is typically fixed, such that if it moves from a metallic to an ionic environment—as happens in corrosion, for example— the potential is unable to capture tbe local change in either oxidation state or bonding that occurs. New classes of potentials have been developed to overcome this restriction. Reactive force fields, in particular, are attracting increasing interest for interfacial problems, due to tbeir inbuilt ability to accommodate changes in coordination and oxidation state [11, 201-203]. 

The development of such advanced toolkits, using either state-of-the-art potentials such as the reactive force field [173] or multiscale modeling projects [11,12,20], is an active focus in our current portfolio of research projects. 

The DFT results for the previous addressed 16 selected processes were used to train a reactive force field (RFF) [17], from which the rates of all 544 relevant processes were calculated for the self-diffusion of gold on bare Au(lOO) surface. In order to investigate the effect of chlorine on Ostwald ripening, the explicit influence of chlorine atom on top of a gold adatom was tackled by means of DFT results for the same 16 selected processes as before. The results will be presented in Section 3.6. 

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. 

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