HOW ACCURATE ARE MOLECULAR MECHANICS PREDICTIONS FOR FULLERENES - A BENCHMARK COMPARISON WITH HARTREE-FOCK SELF-CONSISTENT-FIELD RESULTS

As research into the growing family of fullerene compounds continues t o expand, theoreticians making predictions about these large carbon cl usters are interested in reliable approaches to reduce the computation al expense of calculations. Here we show that an empirical method, mol ecular mechanics (MM3), can be effectively used to optimize the geomet ries of fullerenes and consequently reduce the time required for more elaborate quantum mechanical calculations. Equilibrium structures and heats of formation were predicted for 22 fullerenes ranging from C28 t o C120 using MM3. The MM3 geometries are found to be in good agreement with those obtained by the minimum basis Hartree-Fock self-consistent field (SCF) method. However, the heats of formation obtained with MM3 and SCF are quite different. At the MM3 optimized geometry, an SCF en ergy point was calculated for each structure and found to be very clos e to the fully optimized SCF energy. This procedure yields accurate en ergy differences between isomers at a fraction of the computational co st. We propose other ways of using MM3 to speed ab initio calculations as well.