Different methods of calculations are compared to experiments at 77 K for the broadening Gamma and shift s of some HCO+ rotational transitions by collisions with Argon. Capture method and Anderson, Tsao, and Curnutte (ATC) method with straight line trajectories are both not able to reproduce the trend of Gamma with J. A discussion of the origin of such a discrepancy allows to obtain better insights into the different causes of collisional relaxation. Quite good agreement is obtained by both a hybrid method or ATC calculations using a more realistic translational dynamics. However, some discrepancy still exists for high J values.