The energy of each population for increasing inhomogeneity parameter φ_σ and collision velocity v₀ = 10   m   s^-1 (top) and v₀ = 12.5   m   s^-1 (bottom). The energy of each population is the sum of translational, rotational and vibrational energy and normalised by the initial kinetic energy E_init. The two latter energy contributions are negligible for the head-on collisions presented here. Owing to the strong correlation with mass, this diagram follows a similar trend to Fig. 4. The energy contributions are divided up into E₁, E₂, E_pw, and E_sr for the largest and second largest fragment, the power-law and sub-resolution population, respectively. In both velocity cases, both E₁ and E₂ decrease with increasing φ_σ. In contrast, E_pw and E_sr increase. In particular, the high fraction of kinetic energy stored in the sub-resolution population is remarkable.