As can be seen from table , several combinations achieve a remarkable precision when computing the experimental transition temperature (T 1/2 = 303 K), as has been previously reported. An optimal balance between computational cost and accuracy can be achieved using OLYP/BS4. The same is observed by analyzing the metal-ligand bond lengths (see Supporting Information), as Mn-C bond lengths are in excellent agreement with the experimental data (d(Mn-C) = 2.409 and 2.121 Å for high and low-spin, respectively). Slightly better results are obtained with OLYP/BS5, but the computational cost increases by a factor of 10. Thus, for the rest of this work we will use OLYP/BS4 as a method. Previously, we showed that in the [Mn(Cp n-Me ) 2 ] family (n = 0 to 5), it is possible to tune the ligand field via ring functionalization. Adding more methyl groups to the cyclopentadienyl ring increases the gap between the non-bonding and antibonding orbitals, switching from spin-crossover systems, such as [Mn(Cp) 2 ] or [Mn(Cp As can be seen from Table , in the low-spin state, the average value for d(Mn-Cp) remains almost unaltered, with an average value of (1.740  0.002) Å (95% confidence). The same trend is observed for the high-spin state as well ((2.090  0.004) Å, 95% confidence). However, although the geometry remains almost identical, large changes in the spin-state energy gap as well as the T 1/2 are observed. These changes can therefore only be attributed to the changes in the electronic structure of the [Mn(Cp 1-R ) 2 ] system introduced by the R group.