The phase diagram δ/r versus ϕ_g using the first set of parameters in Table 2. Triangles are in the gas state, and circles are in the cluster state. The red solid fitting curve is a best fit with Eq. (11), in which the restitution coefficient ε = 0.9 and the fitting parameter C₀ = 10.5 (red line). The black and blue dashed lines give a range of fitting for C₀ = 11 (black dashed line), C₀ = 10 (blue dashed line). The three points a, b, and c are the feature points corresponds to Figs. 2(a)–2(c), respectively.