https://doi.org/10.1351/goldbook.S05611
This expression refers to the assumption, essential to conventional
transition-state theory
, that the energy of the system may be expressed as the sum of components associated with different degrees of freedom
. In transition-state theory it is assumed that the energy of the motion of the system through the dividing surface
of the potential-energy (reaction) surface
is separable into various components. In many practical calculations it is assumed that the energy of the system is separable into electronic, vibrational, rotational and translational energy.