A new molecular mechanics force field (called MM3) for the treatment of aliphatic hydrocarbons has been developed and is presented here. This force field will enable one to calculate the structures and energies, including heats of formation, conformational energies, and rotational barriers, for hydrocarbons more accurately than was possible with earlier force fields.In addition to simple molecules, a great many highly strained molecules have been studied, and the results are almost always of experimental accuracy.Over the last 20 years or so, molecular mechanics1 2"3 has developed into a standard powerful method for studying molecular structure and related properties. When the MM2 force field was first formulated during the 1976-1977 period,3•4 all of the heat (1) (a) Engler, E.

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148.9, 114.7, and 129.2, respectively, so the agreement is good, fair, and poor, respectively. The MM3 values are 148.2, 129.3, and 154.9, which is some improvement.The MM3 force field gives 1.5431 Á for the bond length in diamond (calculated with our usual model of fused adamantane rings7). The MM2 value is 1.5417, and the experimental value is 1.5445. The latter cannot contain rigid body motion and should be approximately an ra value. Thus, the discrepancy may be slightly more serious than it appears.

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