In the present paper we report the key results of a comprehensive computational study aimed at investigating the dependence of the singlet-triplet energy gap in phenyl-bridged bis-nitroxide diradicals, on the basis set and oil soft structural parameters like torsion and pyramidalization. We have compared the BS-DFT technique with the post-Hartree-Fock DDCI2 multireference approach. With this latter method we have also studied the different role that a and pi core and virtual orbitals have in the resulting singlet-triplet energy gap. The results obtained represent one step forward in the defintion of a protocol for an efficient and reliable computation of spin-spin Coupling in diradical systems.