Development of an all-atom force field for heterocycles. Properties of liquid pyridine and diazenes

An approach to the general extension of the OPLS all-atom (OPLS-AA) force field to heterocycles has been explored with testing for pyridine and the diazenes, pyridazine, pyrimidine, and pyrazine. For the non-bonded interactions, the partial atomic charges are obtained from fitting to the electrostatic potential surfaces from ab initio RHF/6–31G∗ calculations and standard OPLS values are used for the Lennard-Jones parameters. The harmonic bond-stretching and angle-bending parameters are largely adopted from the AMBER force field. The resultant OPLS-AA force field is shown to perform well for computing the structures of the heterocycles, heterocycle-water interaction energies, and thermodynamic properties of the four pure liquids. The latter quantities were computed from Monte Carlo statistical mechanics simulations; the average errors in computed densities and heats of vaporization are 0.8% and 2.7%. Free energies of hydration were also calculated for pyridine and pyrazine, and provided errors of under 1 kcal mol−1 in comparison to experimental data.