Molecular mechanics potential energy functions from vibrational spectroscopy and quantum chemistry

The construction of molecular mechanics potential energy functions is discussed from the viewpoint of vibrational spectroscopy. The rapid growth in computer capacity has made it possible to use increasingly higher level quantum chemical methods to derive information about molecular potential energy surfaces and electrostatic properties. If such information is fully utilized it could lead to a breakthrough in the ability of molecular mechanics and dynamics simulations to predict vibrational spectra. Two methods that make extensive use of quantum chemically calculated data are compared. It is concluded that the Spectroscopically Determined Force Field technique, in which scaled ab initio force constants and structures are mathematically transformed into molecular mechanics energy parameters, has several advantages compared to least squares fitting procedures. Torsion potentials are discussed separately, with special attention given to C(sp2)-C(sp2) bonds. Finally, a few existing force fields are compared in their ability to reproduce vibrational frequencies.