The structures and properties of indole and its complexes in S-0 and S-1 were characterized with the Hartree-Fock (HF), configuration interaction with single excitation (CIS), and complete active space self-consistent field (CASSCF) methods, using the 3-21G and 6-31G* basis sets. The influence of the intermolecular interaction on the structures and properties of the complexes was discussed in detail. A comparison is made between the results reported here and those from previous theoretical and experimental studies.