ReaxFF method

Take hydrodesulfurization catalysis, for example. We used an active Ti02 as the catalytic support to load the active species for catalysis. Till now, there are many works devoted to research the hydrodesulfurization catalysis mechanism by DFT and experiment, and the reaction pathway of hydrodesulfurization has been figured out. DFT was almost focused on the simulation of the active site. For example, Raybaud et al investigated the interaction of the reactants and intermediate with the M0S2 or NiMoS catalyst by DFT (Dupont et al, 2011). Also, they studied the interaction between active species and support (Costa et al, 2007). However, there was litde work on the interaction of reactants or resultants with catalytic support. The ReaxFF method can be apphed to investigate the reaction on Ti02. It can combine the effect of surface and the effect of transport into reaction process so that the reaction time and the transport time can be separately studied to find out the key influence factor. 

In this particular ReaxFF method, more detail information is initially obtained from quantum mechanics that is used to parameterize ReaxFF for the system of... 

The authors used the optimized structure to conduct RMD simulations in which hydrocarbons interact with the catalyst. The optimized catalyst displays several channels in the oxide through which hydrocarbons can diffuse to reach active sites. This particular simulation reveals that propane can diffuse through the C72-labeled channel in the oxide, but not through the C7i-labeled channel. Such observations can reveal structural factors that influence catalyst selectivity, because different hydrocarbon species will tend to diffuse through different channels. This property can be exploited to design highly selective MMO catalysts. This result demonstrates the applieability of the ReaxFF method for characterizing complex metal-oxide eatalysts, whieh ean be readily extended to supported metal-cluster/metal-oxide eatalysis. 

Buehler et al. presented a preliminary study on formation of water from molecular oxygen and hydrogen using a series of atomistic simulations based on ReaxFF MD method.111 They described the dynamics of water formation at a Pt catalyst. By performing this series of studies, we obtain statistically meaningful trajectories that permit to derive the reaction rate constants of water formation. However, the method requires calibrations with either ab initio simulation results in order to correctly evaluate the energetics of OER on Pt. Thus, this method is system specific and less reliable than the ab initio methods and will not replace ab initio methods. Nevertheless, this work demonstrates that atomistic simulation to continuum description can be linked with the ReaxFF MD in a hierarchical multiscale model. 

Goddard et al.18 are currently carrying out exploratory calculations on both OER and proton conduction processes using the ReaXFF MD method. They anticipate that this will establish a very realistic MD model of a tiny fuel cell and will allow determining how the performance changes as the composition, configuration, and other conditions are altered. 

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. 

Fig. 1 Hierarchical multiscale, multiparadigm approach to materials modeling, from QM to the mesoscale, incorporating breakthrough methods to handle complex chemical processes (eFF, ReaxFF). Adapted from our multiscale group site http //www.wag.caltech.edu/multiscale...

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. 

Methods of quantum mechanics (quantum chemistry) were utilized to predict the products of thermal decomposition of triacetone triperoxide (TATP) [30], i.e. oxygen, acetone and unstable acetone monoperoxide. By means of a ReaxFF monomolecular molecular-dynamic cookoff simulation it was found [31] that the initiation of TATP decomposition in the condensed phase is dominated by a monomolecular process [31]. This simulation demonstrates that this initiation is entropy-driven, since the initial reaction is almost energy-neutral [31]. 

Traditional MD simulations can investigate the different residence times due to the hydrophobic property of the surface of the pores and then obtain different surface fluid transport coefficients and provide a reliable basis for the experimental design. Quantum chemical calculations can consider reactive molecules with different reaction mechanisms for the active surfice of pores the reactivity will also affect the transport fluid to some extent. MD simulation cannot reflect the chemical interaction between the particles and cannot simulate a chemical reaction, while quantum chemical calculations cannot be apphed to large systems. ReaxFF simulation method is a new simulation method rising in recent years it can simulate not only the transport process but also chemical reactions and can simulate transport and reaction in a large system at the same time, which can fill the gap between quantum chemistry and classical force field (empirical force field). It simulates the process with more reahstic method because we can not only obtain the transport properties of the fluid but also reflect a chemical reaction of fluid and the surface. 

The most basic approach to carry out MD simulations for larger systems is to use classical force fields. A variety of different force fields for molecular mechanics (MM) simulations has been developed,which are mainly intended to describe the non-reactive dynamics of large systems. In particular in the field of biochemistry force fields play an essential role to study the complex properties of large biomolecules. However, classical force fields require the specification of the connectivity of the atoms. Therefore, they are not able to describe chemical reactions, i.e., the making and breaking of bonds. To describe reactions, they can be combined with quantum mechanical (QM) methods in so-called QM/MM simulations. In recent years also reactive force fields , e.g. ReaxFF, have been introduced, which overcome this limitation. However, these reactive force fields are typically highly adapted to specific systems by analytic terms customized to describe e.g. certain bonding situations, and only a few applications have been reported so far. 

ReaxFF, a reactive force-field approach, has been used to model the iminium-enamine conversion in the proline-catalysed self-aldol of propanol. " " Quantum mechanical methods have been used to study the same step in the proline-catalysed aldol. " ... 

Table 4 Relative energies (in kcal/mol) of optimized structures of several intermediates with different QM (M06-2X/6-31 -t- G(d,p) and EF (ReaxFF and MMFF94) methods...

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