A Polarizable and Transferable PHAST N2 Potential for Use in Materials Simulation.

A polarizable and transferable intermolecular potential energy function, potentials with high accuracy, speed, and transferability (PHAST), has been developed from first principles for molecular nitrogen to be used in the modeling of heterogeneous processes such as materials sorption and separations. A five-site (van der Waals and point charge) anisotropic model, that includes many-body polarization, is proposed. It is parametrized to reproduce high-level electronic structure calculations (CCSD(T) using Dunning-type basis sets extrapolated to the CBS limit) for a representative set of dimer potential energy curves. Thus it provides a relatively simple yet robust and broadly applicable representation of nitrogen. Two versions are developed, differing by the type of mixing rules applied to unlike Lennard-Jones potential sites. It is shown that the Waldman-Hagler mixing rules are more accurate than Lorentz-Berthelot. The resulting potentials are demonstrated to be effective in modeling neat nitrogen but are designed to also be useful in modeling N2 interactions in a large array of environments such as metal-organic frameworks and zeolites and at interfaces. In such settings, capturing anisotropic forces and interactions with (open and coordinated) metals and charged/polar environments is essential. In developing the potential, it was found that adding a seemingly redundant dimer orientation, slip-parallel (S), improved the transferability of the potential energy surface (PES). Notably, one of the solid phases of nitrogen was not as accurately represented energetically without including S in the representative set. Liquid simulations, however, were unaffected and worked equally well for both potentials. This suggests that accounting for a wide variety of configurations is critical in designing a potential that is intended for use in heterogeneous environments where many orientations, including those not commonly explored in the bulk, are possible. Testing and validation of the potential are achieved via simulations of a thermal distribution of trimer geometries compared to analogous high level electronic structure calculations and molecular simulations of bulk pressure-density isotherms across the vapor, supercritical, and liquid phases. Crystal lattice parameters and energetics of the α-N2 and γ-N2 solid phases are also evaluated and determined to be in good agreement with experiment. Thus the proposed potential is shown to be efficacious for gas, liquid, and solid use, representing both disordered and ordered configurations.