From undergraduate physical chemistry, it has always been tacitly assumed
that the energy of a molecule can be separated into translational, rotational,
vibrational etc. energies to a 'first approximation'.
As such, a molecule has 3N-6 or 3N-5 vibrational modes which are
supposedly completely separable from all other degrees of freedom.
Generally, all of the motions are not completely separable, but can
be minimized by carefully choosing an appropriate coordinate system.