1 with D = 0, i.e. the Heisenberg limit. We carried out these calculations using a series of values of U eff to ensure consistency of behaviour (Fig. ). For our optimised value of U eff = 3 eV, we obtained a self-consistent set of superexchange interactions of J 1 = -2.53(5) meV, J 2 = 0.30(5) meV, J 3 = -0.09(5) meV. To test the robustness of our DFT+U calculations, we performed hybrid calculations using fraction of Fock exchange as implemented in the HSE functional while maintaining a U eff = 3 eV. HSE calculations are computationally expensive due to the calculation of Fock exchange and require the use of norm-conserving pseudopotentials within CASTEP, which limited the sampling of the Brillouin zone and our ability to explore geometry optimisations. Nevertheless, we found that using the HSE functional comparable exchange interactions J 1 = -2.39(1) meV, J 2 = 0.46(1) meV, J 3 = -0.15(1) meV. These energies are comparable in magnitude to those found experimentally for CrCl 2 (pym), but are notably larger, likely due to the unphysically large degree of delocalisation.