Notably, the simpler versions of the above approaches, which use a smaller number of free parameters resulted in better predictions in most cases. One of the main results of this paper is the successful demonstration that rational design of functional molecules by quantum-chemistrybased ligand engineering is indeed possible using fast and affordable calculations. The studied system represents a model molecular switch, where the small size and the subpicosecond light switching of the magnetic and optical properties possess great potential for various applications. In a broader context, we have demonstrated that by identifying the most relevant position for substitution and selecting suitable electron donating substituents it is possible to reshape the potential energy surfaces of the participating electronic states and thereby achieve significant changes in the dynamics using fast and accessible quantum chemical tools. We previously published very accurate potential energy surfaces calculated for the ground and excited states of the iron bisterpyridine complex using CASPT2. However, those calculations are both expensive and require significant expertise, therefore they are not yet feasible to serve as feedback for the purpose of ligand engineering, where a large number of compounds (e.g. different substituted derivatives of a parent compound as in our case) need to be screened and evaluated. In this case faster, simpler and cheaper methods are needed, and as our results convincingly demonstrate DFT with an appropriate exchange-correlation functional is a reasonable choice to obtain ground and excited state energies. Additionally, if high-level PES are available from the literature for a closely related compound (e.g. the parent compound of a series of substituted derivatives), a crude (but in our case surprisingly accurate) approach is to calculate the minima of the states for the other compounds, and simply shift these accurate PES to these values and read off the barrier heights from the resulting graph. This computationally undemanding approach to determine crossing however, is closely related to the well-known inverse energy gap law in the SCO-literature; 7 it uses the quintet-singlet energy difference, but goes beyond it by making an estimate to the barrier. While crude, this approach can be carried out for a set containing several tens of related compounds with minimal effort. While thereby the zero point vibrational energy as well as SOC couplings are neglected, this present study demonstrates that the excited state lifetimes so obtained are qualitatively rather accurate. With a moderate increase in computational complexity, however, these properties can also be taken into account based on efficient DFT calculations thus coming closer to quantitative precision in the prediction of the excited state lifetimes.