A COMPARISON OF CONFORMATIONAL ENERGIES CALCULATED BY MOLECULAR MECHANICS (MM2(85), SYBYL-5.1, SYBYL-5.21, AND CHEMX) AND SEMIEMPIRICAL (AM1 AND PM3) METHODS

Conformational energies of different conformers have been calculated for a series of molecules using various molecular mechanics and semiempirical methods. The quality of the force fields has also been tested by calculating barriers to rotation about carbon-carbon bonds. The molecular mechanics force fields used are MM2(85), Sybyl 5.1, Sybyl 5.21, and ChemX, ver. Jan 89. The semiempirical methods used are AM1 and PM3. Molecules with different functional groups, for which good experimental data exist, have been selected. The semiempirical methods generally calculate barriers to rotation which are lower than the experimentally determined. The conformational energies for hydrocarbons are reasonably well reproduced by all tested methods although MM2(85) gives the quantitatively best agreement with experiments. For compounds containing oxygen, nitrogen and halogens MM2(85) gives results which are in best agreement with the experimentally determined values.