By using molecular dynamics simulation, temperature and density dependences of the power spectra of the velocity autocorrelation functions of acetonitrile have been investigated. Strong temperature dependence was observed only in the low frequencies of less than or equal to30 cm(-1), where the contribution of the diffusive motion was predominant, while the density effect was observed in the whole spectral region of the low-frequency phonon modes. Not only the velocity autocorrelation functions but also the velocity and the angular velocity cross correlation functions were calculated to elucidate the liquid dynamics. The amount of the angular velocity cross correlation was smaller than that of the velocity cross correlation. Oscillations were observed for angular velocity cross correlation functions, which clearly indicate that a nonnegligible portion of the initial angular momentum of the central molecule transferred to the surrounding molecules was given back to the central molecule for some time before the eventual spread over the system. We found that the angular velocity cross correlation function between the central molecule and the molecules in the outer half of the first coordination shell is negative in the short times, which suggests that the molecules move like a pair of coupled gears. Such molecular motion may be important for the diffusion process in the liquid.