The traditional procedure for implementing VTST assumes that rigid-rotor harmonicoscillator (RRHO) approximations (with corrections, such as hindered-rotor assumptions for intermolecular bending modes) are valid along a minimum-energy reaction path (RP), and relies on determination of normal-mode harmonic frequencies and moments of inertia along the reaction path. This approach is thus known as RP-VTST. For radical-radical reactions, the transition state typically lies at separations where the radical-radical bending motions are not well described as either harmonic oscillators or free rotors. An alternative procedure called "flexible TST" uses an approximate separation into what are essentially "conserved" vibrational modes of the radicals and the remaining "transitional" modes describing the relative and overall rotations of the radicals. This separation allows for a fully-coupled anharmonic treatment of the radical-radical bending and rotational degrees of freedom. Within this approach, the number of energetically accessible quantum states of the transition state for a given energy E and total angular momentum J, N ‡ (E, J), can be evaluated as a convolution of a direct quantum sum for the conserved mode contribution with a classical phase-space-integral (PSI) treatment of the transitional 5 mode contribution. This approach is popular for its accurate treatment of issues involving low-frequency modes. However, in its original formulation, it assumes that the reaction coordinate is defined by the radical-to-radical center-of-mass separation distance, which fails at short separations, especially when the reactive site is far from the center-of-mass within a given fragment. The variable-reaction-coordinate (VRC) approach represents an improvement on the center-of-mass-based flexible TST by incorporating variation in the reaction coordinate definition as well. In other words, both the value and the definition of the reaction coordinate are variationally optimized, affording superior descriptions of the reaction coordinate at short fragment separation, while retaining the classical PSI treatment of transitional mode contributions. Monte Carlo sampling of the transitional modes and subsequent integration of the classical PSI representation of the transition-state partition function, are commonly carried out via direct dynamics (i.e., with "on-the-fly" potential evaluations of the sampled geometric orientations). The optimal dividing surface at a given temperature T is obtained by variationally minimizing the resulting fragment association rate coefficient with respect to both the value and the definition of the generalized reaction coordinate.