Within the framework of static, state-of-the art quantum chemistry, there is no straight forward way of calculating the onset of entropy in a (barrierless) reaction, where the electronic energy monotonously increases upon fragment separation. However, in reality the two fragments start to rotate and translate upon separation, resulting in an increase in translational and rotational entropy and consequently, a barrier in the Gibbs free energy along the reaction path. From an experimental point of view, the reaction can be understood to be controlled by diffusion. In theory, such transition states are also referred to as "loose" transition states and are characterized by a free or almost free rotation of the two fragments with respect to each other. Most accurately, the free energy would be obtained from repeated sampling of the reaction path using ab initio molecular dynamics and subsequent entropic path sampling. However, this strategy is computationally very demanding even for medium sized molecules 2 and finding appropriate solutions to adequately approximate the entropy in chemical reactions is one of the remaining challenges in computational chemistry. In modern quantum chemical methodologies, the thermal and entropic contributions are calculated by approximating the partition function q, translating one molecule into an ensemble at finite temperature and pressure, q = ∞ i g i e ε i kT (1) with ε i being the energy of the microstate i and g i being the degeneracy factor. The partition function q is separated into rotational, translational, vibrational and electronic contributions, assuming an ideal gas, rigid rotor, and harmonic oscillator model (RRHO). The resulting q can be used to calculate the entropy,