The structure of nonlinear optical crystal benzimidazole is confirmed by powderX-ray diffraction method. The optimization of the structure has been done by densityfunctional theory (DFT) with B3LYP level and 6-311G++(d,p) basis set. The fundamentalvibrational frequencies of benzimidazole (BZE) are calculated using density functionaltheory (DFT) methods at different levels of calculation. The HOMO-LUMO energies havebeen investigated which proposes that the probability of charge transfer to take placeinside the molecule. Mulliken population study on atomic charges was also calculated.