Several DFT methods were tested for the [Mn(Cp 1-Me ) 2 ] system, including TPSSh, OPBE, OLYP, M06L, 34 B3LYP, 35 B3LYP* and SCAN. A full optimization in both high-and low-spin state using BS3 was done, followed by the corresponding vibrational analysis to ensure its minimum nature. The corresponding spin-state energy differences as well as relevant geometric parameters and the calculated T 1/2 (where meaningful) are given in table . Mn-C bond lengths compared with the experimental values as well as an appropriate energy gap for SCO to occur. Among the most promising methods, a systematic exploration of the basis set effect was carried out. A total of five different basis set schemes were tested (BS1 to BS5, see computational details). The results are summarized in Table .