Force field methods reactive energy surfaces

Computer experiments particularly use quantum chemical approaches that provide accurate result with intense computational cost. Classical or semiempirical methods on the other hand are able to simulate thousands or up to millions of atoms of a system with pairwise Lennard-Jones (LJ)-type potentials [104-107]. Thus, LJ-type potentials are very accurate for inert gas systems [108], whereas they are unable to describe reactions or they do so by predetermined reactive sites within the molecules of the reactive system [109]. van Duin and coworkers [109-115] developed bond-order-dependent reactive force field technique is called ReaxFF as a solution to the aforementioned problems. Therefore, ReaxFF force field is intended to simulate reactions. They are successfully implemented to study hydrocarbon combustion [112,115,116] that is based on C-H-0 combustion parameters, fuel cell [110,111], metal oxides [117-122], proteins [123,124], phosphates [125,126], and catalyst surface reactions and nanotubes [110-113] based on ReaxFF water parameters [127]. Bond order is the number of chemical bonds between a pair of atoms that depends only on the number and relative positions of other atoms that they interact with [127]. Parameterization of ReaxFFs is achieved using experimental and quantum mechanical data. Therefore, ReaxFF calculations are fairly accurate and robust. The total energy of the molecule is calculated as the combination of bonded and nonbonded interaction energies. 

Figure 3.3 ReaxFF reactive force fields are developed to reproduce DFT data for complex chemical reactions, such as heterogeneous catalysis. In the top panel, a comparison of a calculated ReaxFF reaction energy profile (dashed line) is compared to the corresponding DFT results (solid line) for a model catalytic reaction, CH4 + O2 ->CH20 + H2O on a small V4O10 cluster. The method allows modelling of complex chemical reactions on picosecond or longer time-scales to be carried out for large simulation cells involving thousands of atoms or more under realistic temperature and pressure conditions. The lower panel shows a snapshot from a simulation of VO c catalysis of complex hydrocarbons using a force field for hydrocarbon catalysis on vanadium oxide surfaces. ...

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