ABSTRACT
A force field was developed to integrate a new reactive potential form within molecular dynamics (MD) that is defined as RPMD. This force field is based on transition state theory and considers bond breaking and forming that can be applied to simulate chemical reactions using MD techniques. We report the force field, CRACK which splines together a harmonic well and a shallow Lennard-Jones well: the harmonic well models C-C covalent bond while the Lennard-Jones well models long-range van der Waals forces and these two wells are separated by an energy barrier which controls dissociation and recombination, for studying n-alkane pyrolysis processes and compare to experimental trends. To validate the reactive potential form RPMD as well as the force field CRACK, the dissociation probabilities and product distributions of two n-alkane systems were analyzed and their time history of pyrolysis was illustrated. The research on dissociation probabilities shows that the defined pyrolytic temperature could be altered by changing the displacement constant de which controls the height of dissociation barrier of CC bond and it was also found that the pyrolytic temperature of n-octane was higher than of n-decane at the same de, which is consistent with the pyrolysis fundamental. The time history of decane pyrolysis revealed an inspirer result that the CRACK force field certainly reflected the recombination between free radicals. Another exciting result is that, by analysis of the distribution of pyrolytic products, the main products distribution was observed to transfer from mid-short to mid-long chains and chain distribution tended to shorter or longer with the elevated temperature which is well consistent with the regularity of thermal cracking. We announce that molecular dynamics can be certainly used to simulate the process of chemical reaction as long as the reactive potential is proper.
