The first assumption implies that we will not describe strongly anharmonic modes, like intramolecular hydrogen bonds or internal rotations (like those methyl groups are prone to). We can consider an isolated molecule as an ensemble of weakly coupled quantum harmonic oscillators. By "weakly coupled," I mean that energy can flow between vibrational modes, but the vibrational energy is still approximately given by Eq. ( ). This energy flow is crucial for the molecule to ensure that all microstates sharing the same energy are equally probable. For instance, suppose the molecule has 1 = 300 cm -1 and 2 = 600 cm -1 , a microstate with n1 = 2 and n2 = 0 should be as likely to occur as another one with with n1 = 0 and n2 = 1 (assuming that nk for k > 2 is the same in both microstates). Although a weak coupling is assumed, it is never explicitly considered here.