where τ -1 Debye is the relaxation rate, ω is the angular frequency of the AC field and χ T and χ S are the isothermal and adiabatic susceptibilities, respectively. The α parameter (0 < α ≤ 1) describes the width of the distribution of relaxation times, with α = 0 corresponding to an infinitely sharp distribution (the Debye model). AC methods typically have a lower frequency limit of ca. 0.1 Hz, and therefore cannot accurately characterise SMMs with rates slower than ∼ 1 s -1 .