Lennard-Jones parameters for combustion and chemical kinetics modeling from full-dimensional intermolecular potentials

Lennard-Jones parameters for use in combustion modeling, as transport parameters and in pressure-dependent rate-coefficient calculations as collision rate parameters, are calculated from accurate full-dimensional intermolecular potentials. Several first-principles theoretical methods are considered. In the simplest approach, the intermolecular potential is isotropically averaged and used to determine Lennard-Jones parameters. This method works well for small species, but it is not suitable for larger species due to unphysical averaging over the repulsive wall. Another method considered is based on full-dimensional trajectory calculations of binary collisions. This method is found to be very accurate, predicting Lennard-Jones collision rates within ~10% of those obtained via tabulated (experimentally-based) Lennard-Jones parameters. Finally, a computationally efficient method is presented based on one-dimensional minimizations averaged over the colliding partners’ relative orientations. This method is shown to be both accurate and efficient. The good accuracy of the latter two approaches is shown to be a result of their explicit treatment of anisotropy. The effects of finite temperature vibrations and multiple conformers are quantified and are shown to be small. The choice of potential energy surface has a somewhat larger effect, and strategies based both on efficient semiempirical methods and on first-principles direct dynamics are considered. Overall, 75 systems are considered, including seven baths, targets as large as heptane, both molecules and radicals, and both hydrocarbons and oxygenates.