As part of a program to investigate the linear and nonlinear susceptibilities of acetonitrile in the condensed phase, we report on the accurate calculation of the molecular electric properties of acetonitrile, taking into account geometry and basis set effects, static and dynamic electronic correlation, vibrational contributions, and frequency dispersion. All correlated single reference state methods as well as the multireference SCF with a Moller-Plesset second-order perturbation correction (MRMP2) yield similar values for the electronic contribution to the polarizability alpha and the second hyperpolarizability gamma. For the first hyperpolarizability, however, differences between the highly correlated methods CCSD(T) and MRMP2 remain. Vibrational contributions to the electric properties are calculated analytically and using two numerical finite difference methods at the Hartree-Fock level and at the correlated second-order Moller-Plesset level using finite field difference methods. Basis set convergence and convergence with the level of anharmonicity are examined. Computed values of the quantity mubeta(parallel to)(-2omega; omega, omega)/(3kT) + gamma(av)(-2omega; omega, omega, 0) agree with temperature-dependent experimental values at two different frequencies within 10%. Using the highest correlated methods, liquid-phase susceptibilities are computed in the dipolar Onsager reaction-field approximation. Excellent agreement with experiment for the relative permittivity and the refractive indices is found as well as acceptable agreement for the nonlinear susceptibility.