possible negative hubbard U in potassium doped p-terphenyl

Recently, a possible organic superconductor, potassium doped p-terphenyl, which was recently reported to exhibit transition temperatures in the order of 120~K, draws people's interests. By optimizing the crystal structure of KC₁₈H₁₄, we noticed that half of the p-terphenyl molecules are nearly coplanar while another half of p-terphenyl molecules are still tilted, while our calculation about molecular K_xC₁₈H₁₄ shows that benzene rings are almost coplanar in K₂C₁₈H₁₄ but tilted obviously in C₁₈H₁₄. This intriguing phenomenon indicates that there might be a negative hubbard U in potassium doped p-terphenyl. Through mean field theory, we shows that superconductivity occurs in this possible neagtive-U system at low temperatures.