The first thousand-molecule atomistic simulation of a mesogenic system is described. The simulations involve a study of the mesogen 4-(trans-4-n-pentylcyclohexyl) benzonitrile (PCH5) in the isotropic phase, close to the phase transition to a nematic liquid crystal. An all-atom force field is, employed to model accurately both internal and external degrees of freedom. Results are presented for the system density and translational diffusion, for molecular structure in the liquid, and for dipole and orientational correlation. While the system size has not reached the thermodynamic limit, most of the calculated properties of the,system are in good agreement with experiment and the predictions improve dramatically from simulations of a smaller 125 molecule system.