Ab initio MO study of 1,5-hexadiene at the MP2/6-31G* level shows that, unlike n-butane, there is little energy difference between the anti and the gauche conformations. However, molecular modeling using either MM2 or MM3 gives an energy that is too high for the gauche conformations of 1,5-hexadiene. The overestimation of the energy for the gauche conformations in 1,5-hexadiene was identified as the reason for the failure of molecular mechanics in predicting conformational energies of compounds involving the 1,5-diene-3,4-diol structural unit.