In spin crossovers, one can typically leverage the fact that the SOC is small enough for Fermi's golden rule to be valid. Although conceptually straight-forward, this quantummechanical approach requires the calculation of the vibrational wavefunctions corresponding to each diabatic state in order to obtain the Franck-Condon factors. This is unfeasible for all but the very smallest molecular systems. Thus, the method can only be applied along with strong assumptions and simplifications, typically a global harmonic approximation. These approximations do not appear to be valid as studies that use these methods predict results which are several orders of magnitude smaller than the experimental lifetimes. Even including anharmonicities in certain modes or rotational effects does not solve the problem. An alternative approach that avoids the explicit computation of wavefunction overlaps is instead based on a nonadiabatic generalization of transition-state theory (NA-TST). This method depends solely on an analysis of the reactant minimum and the minimum-energy crossing point (MECP), which plays the role of the transition state in nonadiabatic reactions. It has been used to study spin-crossover reactions in a number of molecules. It is common to employ a one-dimensional tunneling correction, known as the weak-coupling (WC) approximation, which is based on the assumption that the diabatic states are linear functions of the predetermined reaction coordinate. This calls the qualitative picture of the reaction mechanism into question as it ignores the true shape of the PESs as well as the intrinsic multidimensional character of nuclear tunneling. Recently Lykhin and Varganov employed these methods to study the spin crossover of thiophosgene and suggested that the approximate treatment of the tunneling was the main cause of the discrepancies they observed. Since tunneling is expected to be a key factor in many spin-crossover reactions, there is clearly a high demand for new theories which incorporate the effects of multidimensional tunneling and reveal the mechanism in an intuitive way. As we cannot solve the vibrational Schrödinger equation for the full problem, we would like to approximate Fermi's golden rule as accurately as possible without making drastic simplifications for the shape of the diabatic surfaces. This can be accomplished using golden-rule instanton theory, which is an extension of the original instanton method for reactions within the Born-Oppenheimer approximation. The theory constitutes a rigorous semiclassical approximation to a path-integral formulation of Fermi's golden rule and has been shown to be in almost perfect agreement with the quantum rate for a number of multidimensional and anharmonic model systems. The method locates the ideal tunneling pathway for each temperature in a self-consistent fashion, which enables one not only to compute accurate reaction rates but also to predict temperature-dependent changes of the mechanism.