Relative errors of the potential and forces for the Hernquist models with different particle numbers at r = 0.01 when using the HO method. In each panel, the red, blue, and green lines are the results for HO method with (n_max, l_max) = (16,12), (12,12), and (6,4), respectively. The filled circles are the mean value from 40 random samples, and the error bar is the standard error from these samples.