Fig. 9. Field (left, at 1.8 K) and temperature (right, at 0.45 T) dependences of the average relaxation time for 2 estimated from the Fig. 6 and 7. The red lines are the best fits obtained with the theoretical approach developed in the text. Inset: lowest two Kramers doublets and ab initio computed relaxation mechanism with the MOLCAS code (CASSCF + RASSI level). The thick black lines are Kramers doublets shown as a function of their magnetic moment, M_z, along the main anisotropy axis (z). The green arrows correspond to the quantum tunnelling mechanism (QTM) of ground and first excited states while purple arrow shows the hypothetical Orbach relaxation process. The red arrow indicates the transition between the ground and first KDs. The values close to the arrows indicate the matrix elements of the transition magnetic moments (above 0.1, an efficient spin relaxation mechanism is expected). Thus, this figure highlights that the QTM through the Kramers doublet ground state is dominating the relaxation process at low temperatures.