Eq. 1 and 2 were taken from the work of Ochterski ( uploads/dl/thermo.pdf). While this standard model predominates quantum chemistry, there are inherent limitations due to the nature of the approximated partition function. Efforts have been made to improve the description of the translational entropy by introducing the free volume, taking into account the accessible volume of a molecule in the condensed phase and to improve the vibrational entropy. The latter relies on the calculation of harmonic frequencies, which are increasingly incorrect for low-modes with < 100 cm -1 . Hence, these can either be described by a free rotor model or by shifting these modes to 100 cm -1 , 12 as well as scaling of vibrational frequencies to ensure accuracy. These estimates are commonly referred to as quasi-harmonic approximations. In recent years, the model has been augmented to account for conformational entropy, derived from conformational sampling tools or from classical molecular dynamics simulations. The RRHO model in conjunction with transition-state theory can only account for translational and rotational contributions of the system as a whole, it does not allow for onset of rotation and translation of the fragments upon separation, and it can typically only deal with stationary points on the PES (see Supporting Information, Figure and). A notable exception to the latter is the Single Point Hessian method by Spicher and Grimme that allows the evaluation of the vibrational frequencies through the Hessian at non-equilibrium points by employing a biasing constraining potential to restrain the initial geometry. Another strategy to calculate the vibrational entropy at non-stationary points along the reaction path is the frequency projection method that ensures that the vibrational frequencies remain perpendicular to the reaction path and to the rotational motions. However, this requires the usage of curvilinear coordinates to obtain meaningful results or otherwise will result in jumps in the free energy. Upon separation of the fragments, the vibrational entropy becomes less favourable (and decreases) because six vibrational degrees of freedom will merge into three rotational and three translation degrees of freedom. Once these vibrational frequencies fall below a threshold, they must be treated as rotations and translations. While the frequency projection and the Single Point Hessian methods both account for the decrease in vibrational entropy, only the latter accounts for the correct description of low modes with a (hindered) rotor approximation. At this point, even if vibrational entropy is treated correctly and emerging rotational modes are described by a hindered rotor, the RRHO model is still not able to account for onset of translational entropy and only approximates rotational entropy. In addition, to gain multiple configurations along the reaction path need expensive Hessian matrix computations, despite recent efforts aiming for a computation at a lower level of theory. An extension to conventional TST is variational transition state theory (VTST), where the transition state is determined by variationally optimizing the dividing surface between educts and products so that the net reaction flux is minimized. While VTST has originally been developed for systems with a well-defined barrier in the PES, it can be extended to "loose transition states" and to localize variational transition state for such reactions. However, such calculations require massive Monte Carlo sampling and are therefore very expensive. Due to these limitations and because diffusion controlled reaction steps are common in a chemistry, a different strategy has emerged: Some years ago, McMullin and coworkers encountered a ligand dissociation for a palladium complex. The reaction barrier for this reaction step has been estimated experimentally to be close to the diffusion limit, corresponding free energy barrier of approximately 20 kJ mol -125,26 in most common solvents and this value has widely been accepted as a rule of thumb barrier for such reactions.