Abstract Accuracies of classical, semiclassical and quantum methods are comprehensively examined in calculations of impact line widths of C 2H 2 molecules perturbed by Ar and He. The field of comparative study covers both infrared absorption and Raman scattering lines of acetylene having rotational quantum number J=0–30 at temperatures 173 and 296 K. Calculations have been made by fully classical method and by three basic least approximate semiclassical methods, namely, Neilsen–Gordon (NG) method, peaking approximation (PA) and Smith–Giraud–Cooper (SGC) method. Most accurate ab initio potential energy surfaces (PES) of Yang et al. (1996)  and Mozsynski et al. (1995)  have been applied to model C 2H 2–Ar and C 2H 2–He interactions. The comparison has been made also with available experimental data and with the results of rigorous fully quantum-mechanical calculations within close coupling and coupled states approaches in identical conditions. Semiclassical methods are proved to be not so much accurate as it is generally believed since all they gave in the cases considered seriously underestimated results. The fundamental issue of the adequacy of simplified trajectories in collision broadening calculations is finally reasonably solved. In cases of C 2H 2–Ar and C 2H 2–He systems the use of the “exact” isotropic trajectories (i.e. driven only by the isotropic part of PES) is the main reason of failing of NG, PA and SGC methods. Thus the neglecting of back-influence of the RT exchange on the classical path is a principal defect of semiclassical methods. Finally, the application of simplified trajectories is recognized as inadequate and risky in broadening calculations for molecules having relatively small rotational constants when accurate ab initio PES are applied.