The molecular calculations of the hyperfine coupling parameters have been extensively tested in a variety of transition-metal complexes . Electron paramagnetic resonance (EPR) calculations were conducted to find isotropic hyperfine coupling constants for Ni using NiN4C8 and NiN4C10 clusters in ORCA program package (version 5.0.0). We used B3LYP density functional with SV(P) for all elements and TZV(P) basis set for Ni for geometry optimizations. Furthermore, we used def2/J auxiliary basis sets for all elements together with CP(PPP) basis set for Ni used during single point calculations. Finally, calculations of the isotropic hyperfine A tensor (Aiso) and electron densities at the Ni nuclei were performed using the EPR-II basis set and the zero-order regular approximation (ZORA). The spin densities at Ni were calculated using Multiwfn wavefunctional analyzer. For energy comparison, we used spin-polarized DFT calculations for geometry optimization using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional of generalized gradient approximation as implemented in the Vienna ab initio simulation package (VASP). The energy cutoff of the plane wave basis set was 500 eV for all VASP calculations. We applied DFT+U method using Ueff = 6.45 eV for Ni 3d orbital to correct artificial electron selfinteraction. For geometry optimization and formation/binding energy calculation, we used 4 × 4 graphene super cell containing NiN4 (except NiN3, NiN3O1, NiN2O2) moiety in basal plane.