We present a united atom force field for simulations of 1,4-polybutadiene based on ab initio quantum chemistry calculations on model molecules. The geometries and energies of conformers and rotational energy barriers in model alkenes and dienes have been determined from high-level quantum chemistry calculations. A rotational isomeric state (RIS) model for 1,4-polybutadiene based on the conformer geometries and energies of the model molecules has been derived. The characteristic ratio and its temperature dependence for cis-1,4-polybutadiene and trans-1,4-polybutadiene, and the characteristic ratio of a random copolymer of cis and trans units, as predicted by tile RIS model, are in good agreement with experimental values, thereby supporting the accuracy of the quantum chemistry calculations. Torsional potentials fur the united atom force field have been parametrized to reproduce the quantum chemistry conformer energies and rotational energy barriers for rotations about the C(sp(2))-C(sp(2)), C(sp(2))-C(sp(3)), and C(sp(3))-C(sp(3)) dihedrals for the model compounds. The CH2-CH2 united atom nonbonded potential has been taken from previous work on polyethylene melts, while the CH-CH united atom nonbonded potential has been parametrized so as to reproduce the energies of chose conformers of the model molecules involving conformation-dependent second-order interactions. Finally, NPT molecular dynamics simulations have been performed on a melt of 1,4-poly(cis(0.5)-r-trans(0.5)butadiene). and the CH2-CH nonbonded potential has been adjusted so that the experimental melt density of the polymer as a function of temperature is accurately reproduced.