The most stable spin configuration in the Cu(20) unit embedded in the P(8)W(48)O(184) POM framework has been analyzed with ab initio computational methods. Because of the large size of the Cu(20) unit, a fragment approach has been adopted to compute the four possible magnetic interactions. Difference Dedicated Configuration Interaction results reveal that the magnetic coupling between unpaired copper electrons ranges from ferromagnetic (J = 14.8 cm(-1)) to antiferromagnetic (J = -72.6 cm(-1)). With the four calculated J-values we have performed a simulation of the magnetic susceptibility vs. temperature by using the Ising model and a Monte Carlo algorithm to sample the spin configurations. The most stable spin alignment, governed by the most intense antiferromagnetic interaction, features 12 up and 8 down spins. There is no full cancellation of spins that can comply with the magnetic interactions found for this molecule.