For A 2+ , IS3 is always the electronic ground state, except for system size 2, where HS3 is found to be more stable. At system size 5, HS3 is energetically low lying, probably due to the ring open-shell character being stabilized by the extended π system. Since the Mössbauer parameters of HS 3 and IS 3 are indistinguishable within the uncertainty ranges of the isomer shift and quadrupole splitting, the two configurations could probably not be discerned experimentally. Therefore, for model A 2+ , a convergence can be assumed for system size 4. For model B, the electronic ground state converges more slowly. One reason is the change in planarity of the graphene environment: while for system sizes 1 to 3, the graphene plane is planar, it is concave for system size 4 and 5. This means that for structural convergence, at least system size 4 is necessary. At system sizes 4 and 5, all three intermediate spin states are found within 5 kcal mol -1 ).