Ab initio and semiempirical MO calculations were carried out for dimers of p-carboxylate phenyl nitronyl nitroxide, whose geometries were extracted from the X-ray diffraction results. It was shown that the effective exchange integrals for the nearest neighbor dimer are largely positive (ferromagnetic), in accord with the experimental results. The direct potential exchange interactions between SOMOs are operative, giving the large J(ab) values, but the spin polarization plus other higher-order order terms reduce their magnitudes. Implications of these results are discussed in relation to molecular design of high T-c crystalline organic ferromagnets.