The harmonic force field of propane.

The determination of the vibrational force field for a polyatomic molecule is an important, though difficult problem. Typically, there is not enough vibrational frequency information to derive a unique force field experimentally. An alternative approach is to calculate the harmonic force fields using ab initio quantum chemical techniques but the derived frequencies are typically too high by about 5-10% due to the neglect of anharmonicity effects. A successful approach in recent years has been to combine theory and experiment by scaling the calculated force fields slightly to obtain best fits with the experimental frequencies.