A B3LYP/6-31++G(d,p) study of indole dimer was performed. The optimized geometry reveals the existence of N-H . . . pi hydrogen bond in which the benzenoid ring of one subunit acts as a proton acceptor, the interplanar angle between the two monomeric units being 89.4 degrees (a T-shaped structure), with the center-of-mass separation of 6.207 Angstrom. The counterpoise-corrected interaction energy is 2.15 kcal mol(-1) (9.00 kJ mol(-1)). Anharmonic vibrational frequencies of monomeric and dimeric N-H oscillators, their change upon dimerization and the intensity enhancement are excellently reproduced by one-dimensional B3LYP/6-31++G(d,p) vibrational potentials.