Using ab initio calculations all the components of the magnetic anisotropy in a dinuclear [MnIIICuIICl(5-Br-sap)2(MeOH)] single-molecule magnet (SMM) have been computed. These calculations reveal that apart from the single-ion anisotropy, the exchange anisotropy also plays a crucial role in determining the sign as well as the magnitude of the cluster anisotropy. Developed magneto-structural correlations suggest that a large ferromagnetic exchange can in fact reduce the ground-state anisotropy, which is an integral component in the design of SMMs. Zero-field splitting: This study answers the question of whether single-ion anisotropy is always important in the design of single-molecule magnets (SMMs). Ab initio CASSCF studies unfold the role of exchange anisotropy in controlling the effective energy barrier (U eff) of SMMs. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.