We have shown that the zero-field splittings may be predicted computationally with reasonable accuracy even using de novo models, thanks to the predictable coordination environment provided by the mechanical bond. Magnetostructural correlations wi th computational methods previously revealed that the size of D may be tuned through structural changes in [Co II (tbta)N3] + complexes by varying the Lewis basicity of the axial ligand on the N3 -site. In the case of the Co II rotaxane-based complexes presented here, our calculations suggest a new strategy to tune D, based on the control of the intramolecular angles that determine the stability and population ofin this casethe dxy orbital. We have shown that the energy gap between HS and LS states may be estimated even using simple DFT de novo models, although it should be noted that higher levels of theory are required to determine D in these complexes exhibiting multiconfigurational ground states. It is also noteworthy that, using CASSCF, such good agreement between calculated and experimentally obtained D values could be reached with the SCXRD-derived model structures, demonstrating that the structural rigidity and coordination geometry is more important than the exact chemical structure of the ligands. Given that, as we have recently demonstrated, the mechanical bond can enforce unusual coordination environments, including multidentate binding even when the individual ligands are weakly coordinating, there is clearly scope for further ligand engineering. The future of this work lies in investigating rotaxane-based SIMs containing other ions such as Fe II or Ni II with larger total spin values and no nuclear spin to eliminate hyperfine coupling and so improve the efficiency of these nanomagnets.