The presence of two different clusters of 1 in the unit cell, the disordered [CO3]/[OH] and a total of twenty four Cu II ions per cluster poses some problems (over-parameterisation, computational limits) for modelling the magnetic behaviour. In order to overcome these issues, we have constructed three [Cu12] model compounds (Models 1-3, Fig. ) that describe half the cluster, and assume the presence of not more than three distinct exchange interactions based on the different bridging atoms and angles present. The best fit parameters using these models and spin-Hamiltonian (1), where the indices refer to the constituent Cu II ions, µB is the Bohr magneton, g is the g-factor fixed at g = 2.00, Ŝ is a spin operator and Jij is the pairwise isotropic exchange interaction parameter, are collected in Fig. . What is clear is that each model matches the experimental data (Fig. , Fig. ) well but does so with different sets of J values. In other words, there is no unique fit of the susceptibility data and we can only conclude that the J values in 1 range between -56 < J < -139 cm -1 . We disregard Model 2 on account of the erroneously large J2 value. Models based on just 2 J values led to poorer fits and/or erroneously large J values.