Lennard-Jones potential atomic reactions

There have been several studies of the iodine-atom recombination reaction which have used numerical techniques, normally based on the Langevin equation. Bunker and Jacobson [534] made a Monte Carlo trajectory study to two iodine atoms in a cubical box of dimension 1.6 nm containing 26 carbon tetrachloride molecules (approximated as spheres). The iodine atom and carbon tetrachloride molecules interact with a Lennard—Jones potential and the iodine atoms can recombine on a Morse potential energy surface. The trajectives were followed for several picoseconds. When the atoms were formed about 0.5—0.7 nm apart initially, they took only a few picoseconds to migrate together and react. They noted that the motion of both iodine atoms never had time to develop a characteristic diffusive form before reaction occurred. The dominance of the cage effect over such short times was considerable. 

The energy barrier is shown in the Lennard-Jones potential energy graphical representation (Eig. 5.7), indicating how the energy varies with distance between molecules or atoms, as, for example, for the reaction A + B R. 

The existence of these highly mobile quasi-isolated atoms (Fig. 10) could provide new possibilities for catalytic reactions, favoring, for instance, the occurrence of the nonselective cyclic mechanism. Although the potentials used in Hoare and Pal s calculations (Lennard-Jones and Morse-Mye) may be considered as unsuitable for metal clusters, recent calculations (181), made with more realistic potentials, indicate that below 15 A, the clusters with icosahedral symmetry are more stable than the fee cubooctahedra. Unfortunately, the size range in which the polyhedral metal clusters are supposed to be stable does not allow microdiffraction studies. Moreover, when platinum is deposited on an oriented rock-salt face, pseudocrystals with uncommon symmetry are present only in very small amounts, and in a particle size range of 80-120 A, where they obviously result from multiple twinning of fee tetrahedra (182). 

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. 

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