Various mean-field potentials in a model lipid bilayer are calculated by means of molecular dynamics (MD) simulation. The bilayer assembly consists of 200 chain molecules. The anisotropic united atom model is employed for nonbonded interactions and is extended to allow bond length to vary with time. The interfacial translational order is systematically varied and found to correlate strongly with the chain orientational order. A new torsional potential is developed and shown to give order parameters in better agreement with experiment than the Padilla-Toxvaerd potential. Nonbonded interaction reduces the trans-gauche and gauche-gauche transition barriers by 0.9-1.5 kcal/mole. The mean trans-gauche energy difference near the chain tail is close to that in liquid hydrocarbons but 0.34 kcal/mol lower than that in the highly ordered chain region. In contrast to the Marcelja model, both mean intermolecular dispersive and repulsive energies depend exponentially on the chain orientational parameter and the repulsive component has a poor and inverse correlation with the reciprocal of the chain end-to-end displacement along the bilayer normal. Inclusion of spatial heterogeneity effects of the interaction energy, a treatment similar to the Gruen model [Biochim. Biophys Acta 367, 165 (1980)], does not give a better description of the mean intermolecular interaction. A new and unified model for the mean intermolecular interaction energy is developed based on our present MD simulation data. Various possible chain configurations which are responsible for these results are discussed. Finally our MD results suggest that, consistent with the "wobble in a cone" model, a chain molecule can rotate eely within an angular range without being subjected to a strong potential force.