Models of formation and growth of atmospheric aerosols are highly dependent on accurate cluster binding energies. These are most often calculated by ab initio electronic structure methods but remain associated with significant uncertainties. We present a computational benchmarking study of the Gibbs free binding energies in molecular complexes and clusters based on gas phase FTIR spectroscopy. The acetonitrile-HCl molecular complex is identified via its redshifted H-Cl stretching vibrational mode. We determine the Gibbs free binding energy, Delta G(295 K)(o), to between 4.8 and 7.9 kJ mol(-1) and compare this range to predictions from several widely used electronic structure methods, including five density functionals, Moller-Plesset perturbation theory, and five coupled cluster methods up to CCSDT quality, considering also the D3 dispersion correctional scheme. With some exceptions, we find that most electronic structure methods overestimate Delta G(295 K)(o). The effects of vibrational anharmonicity is approximated using scaling factors, reducing Delta G(295 K)(o) by ca. 1.8 kJ mol(-1), whereby Delta G(295 K)(o) predictions well within the experimental range can be obtained.