R as a function of X_H for the best-fit RCN model (Eq. (52), lower solid red line) and DG model (with L = 2, Eq. (75), lower dashed red line); the corresponding values of m_C for each model are shown in the upper “flatter” red lines. Shaded: validity-ranges for the RCN (right) and DG (left) models, darker where they overlap. Thick dash-dotted: mean number of rings per aromatic cluster (N_R/10). Thin dash-dotted: mean number of C atoms per aromatic cluster (n_C/20). Short dashed: aromatic domain size (d_a in nm). Long-dashed: aromatic coherence length (L_a in nm). Diamonds; laboratory data for R (Angus & Jansen 1988, and references therein).