Theoretical calculations were carried out for dimers of 5-methyl-1,2,4-triazole nitronylnitroxide, whose geometry was extracted from the X-ray diffraction results of Kahn and co-workers. It was shown that the effective exchange integrals for the molecular pair are largely positive (ferromagnetic), in accord with the experimental results. The origin of a large ferromagnetic intermolecular interaction in the linear chain structure is successfully identified as SOMO–SOMO direct interaction. Namely, the direct potential exchange interaction between SOMOs is operative, giving a large J ab value. On the other hand, the intermolecular hydrogen bond is not crucial for the ferromagnetic interaction but it contributes to structural stabilization. In addition to semiempirical, ab initio and DFT methods, we developed two types of hybrid DFT methods and successfully applied these methods in order to evaluate the intermolecular magnetic interaction.