Various density functional theory (DFT) methods (BLYP/6-31G^*^*, B3LYP/6-31G^*^*, B3PW91/6-31G^*^*, and SVWN/6-31G^*^*) were used to calculate the equilibrium geometries of indole. On the basis of optimized structures, the harmonic force fields, vibrational frequencies, and infrared intensities were calculated. The average deviations between the experimental and computed frequencies are 16.3, 40.5, 45.1, and 26.4cm^-^1 for BLYP, B3LYP, B3PW91, and SVWN methods, respectively. A normal coordinate analysis was carried out using the BLYP/6-31G^*^* force field. The assignment of the fundamental vibrations for this molecule was performed according to the potential energy distribution (PED).

Key words: INDOLE, MOLECULAR VIBRATION, GEOMETRICAL ISOMERISM