APPLICATION OF A UNIVERSAL FORCE-FIELD TO ORGANIC-MOLECULES

The application of a Universal force field (UFF) to the treatment of organic molecules is described. The ability of the force field to predict the structures of a variety of organic molecules is examined, and the results are compared with the MM2 or MM3 force fields. UFF correctly predicts the structures of unstrained and uncongested hydrocarbons, silanes, alkenes, saturated amines, saturated ethers and phosphines, aromatic systems, and simple unconjugated multiple bond containing compounds such as nitriles, ketones, and imines well. Bond angles are usually correct to within 3-degrees, and bond lengths usually to within 0.02 angstrom. Specifically, the rms error in the UFF predicted C-C bond distances is 0.021 angstrom, with a maximum of 0.067 angstrom for a set of 65 distances. For comparison, the MM2/3 RMS error in C-C distances is 0.012 angstrom with a maximum of 0.029 for the same set of molecules. The UFF rms error in C-N bond distances is 0.024 angstrom, with a maximum of 0.041 angstrom for a set of 13 distances. For the same set of molecules, the MM2/3 rms error in C-N distances is 0.013 angstrom with a maximum of 0.031. The UFF rms error in C-O bond distances is 0.025 angstrom, with a maximum of 0.05 angstrom for a set of seven distances. For the same set of molecules the MM2/3 rms error in C-O distances is 0.007 angstrom with a maximum of 0.015. The ability of UFF to calculate conformational energy differences in simple organic molecules is also examined.