Conformational energy calculations are used to develop a rotational isomeric state (RIS) model for polystyrene including t, g and gBAR states. No adjustment is made to match experimental data as with an earlier two-state model due to Yoon and coworkers. The three-state model correctly predicts a higher limiting characteristic ratio for the syndiotactic polymer as compared to the isotactic. It is shown how molecular dynamics simulations on a periodic cell containing polystyrene fragments in a THETA solvent can be used to make a systematic correction for the short-range solvent effect, enhancing the tendency of the syndiotactic chain to expand relative to the isotactic chain. After solvent correction, C infinity is predicted about 26-28% too low for both isotactic and syndiotactic polystyrene, a deviation that can be attributed to other approximations inherent in the RIS method.