Table ) orientated perpendicular to the coordination plane (0.3° to normal, Figure ). Calculations slightly overestimate the room temperature χT value (Figure ) and replicate a gradual decrease in χT from 300-10 K but do not reproduce the sharp downturn below 10 K or Msat (Figure ). The discrepancies suggests a higher level of mixing in the ground state that would cause a larger splitting in the pseudo-doublet, while the absence of an EPR signal for 2-Tm cannot distinguish between these cases (highly pure EPR-silent ground state vs. pseudo-doublet split by >0.32 cm -1 ). XMS-CASPT2 was computationally prohibitive due to the large number of states for Tm 3+ in an already large molecule.