where κ is the torsional spring force constant, which reflects the curvature of the harmonic potential wells. A similar approach can also be used to determine the torsional spring constants for the C--O bond angles. These distributions are more complex, and, as shown in Figure , the potential energy landscapes are anharmonic. To calculate the force constants, however, we assume that the potential wells behave harmonically in the region between 5 -20°, near the equilibrium bond angles.