A new ab initio potential for the thymine dimer has been developed by using a combination of density functional theory and symmetry-adapted perturbation theory (DFT-SAPT). It has been shown that DFT-SAPT is in very good agreement with the reference CCSD(T) calculations performed for the hydrogen-bonded and stacked thymine dimers. Parallel global cluster optimizations have been carried out employing the new force field from the dimer to n = 50. It has been observed that the resulting cluster structures tend to form the building blocks of the crystal structure of thymine. This study therefore highlights the importance of dimer potentials to gain insights about the crystal structures.