| Abstract
| - As part of a program to investigate the linear and nonlinear susceptibilities of acetonitrile in the condensedphase, we report on the accurate calculation of the molecular electric properties of acetonitrile, taking intoaccount 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 SCFwith a Møller−Plesset second-order perturbation correction (MRMP2) yield similar values for the electroniccontribution to the polarizability α and the second hyperpolarizability γ. For the first hyperpolarizability,however, differences between the highly correlated methods CCSD(T) and MRMP2 remain. Vibrationalcontributions to the electric properties are calculated analytically and using two numerical finite differencemethods at the Hartree−Fock level and at the correlated second-order Møller−Plesset level using finite fielddifference methods. Basis set convergence and convergence with the level of anharmonicity are examined.Computed values of the quantity μβ∥(−2ω; ω, ω)/(3kT) + γav(−2ω; ω, ω, 0) agree with temperature-dependentexperimental 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 agreementwith experiment for the relative permittivity and the refractive indices is found as well as acceptable agreementfor the nonlinear susceptibility.
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