The J values of the Heisenberg model and t and U-eff values in the Hubbard model have been evaluated by the Kohn-Sham density functional theory (KS-DFT) for dimeric pairs of diethyl-spirobiphenalenyl. The Quantum Monte Carlo (QMC) simulation has been carried out with the calculated J values to evaluate the magnetic susceptibility for this molecular crystal, reproducing the experimental tendency. The band gaps estimated by using t and U-eff values are also consistent with the experiment on conductivity.