Hydrazine is a highly reactive liquid base. It is mainly used as a rocket propellant; other uses include but are not limited to, the production of spandex fibers, agricultural chemicals, nickel-plating, nuclear fuel reprocessing, production of plastics and in the removal of halogens from wastewater. Molecular modeling of hydrazine can be a safe and effective method for predicting its chemical behavior during engineering applications including storage and transport, provided accurate molecular parameters are known. These simulations are essential for engineering design and safety analyses of any process that involves hydrazine. In this study ab initio calculations were used to determine the first OPLS-AA force field parameters for bonded interactions in hydrazine, which can be used in extensive molecular simulation studies. Gaussian 03 was used to determine equilibrium geometry data, bond force constants, angle force constants, and torsion parameters with geometry optimizations and single-point energy calculations using second-order Møller-Plesset perturbation theory (MP2), in the 6-31G(d,p) basis. The results are compared with published experimental data and are shown to be in good agreement. Using the updated parameters, Gibbs Ensemble Monte Carlo simulations were conducted to predict vapor-liquid equilibrium for hydrazine.