where ω X i is the frequency for the i-th normal mode (excluding the imaginary frequency associated with reaction-coordinate motion at the transition structure). The CTST and CVT/SCT calculations use the harmonic formula of eq. 6 but with scaled harmonic frequencies; this is called the quasiharmonic approximation. In cases where there is more than one important conformer due to torsional anharmonicity, the separability of rotation and vibration is no longer assumed, and the multi-structural torsional anharmonicity model with a coupled torsional potential is used to calculate coupled conformational-rotational-vibrational PFs (see Computational Details).