We report on the relative conformer energies and rotational energy barriers for meso and racemic 2,4-diphenylpentane (DPP) from ab initio electronic structure calculations. It is found that dispersion interactions between phenyl rings strongly influence the conformational geometries, necessitating the inclusion of electron correlation in both geometry optimizations and energy calculations. Furthermore, basis set superposition contributions to the phenyl-phenyl interactions, estimated by extracting the phenyl rings from the optimized DPP geometries and computing the basis set superposition error for the resulting benzene dimer configurations, are significant. An atomistic molecular mechanics force field is parametrized to reproduce our best values for the conformational energies and rotational energy barriers in DPP obtained from ab initio calculations. Conformational energy contour maps are presented for the DPP enantiomers, and their salient features are discussed. Gas-phase molecular dynamics simulations of DPP have been performed using the quantum chemistry based force field. Important entropic contributions to the conformer populations, due primarily to restricted phenyl group rotation, are discussed.