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The packing structures of thiophene dimers and their effects on excitation energies of thiophene dimers were studied by employing MP2/6-31+G* and TDDFT calculations. Twelve optimized dimers with different orientations were obtained by means of MP2/6-31+G* optimizations. Among them, five T-shaped and three π-stacked thiophene dimers are local minima in energy. The result shows that the preferable conformation of thiophene dimers is the T-shaped packing, which is in agreement with the results in references. All the excitation energies of both T-shaped dimers(5.34-5.48 eV) and π-stacked dimers(5.15-5.18 eV) are lower than that of the isolated thiophene(5.68 eV), indicating that inter-ring interactions decrease the excitation energies.