Carbon atom ratio R as a function of X_H. Dotted line: basic RCN model (Eq. (7)), short-dashed-dotted line: C₂sp² clusters (Eq. (14)), long-dashed line: all sp² carbon atoms in C₆ aromatic clusters (Eq. (35)) and solid line: multi-C₆ ring aromatic clusters (Eq. (40)) and methyl groups (Eq. (52)). The upper and flatter solid lines show the mean atomic coordination number, (dashed), and the mean carbon atom coordination number, (solid) for the full model. Diamonds and plus signs [crosses]; laboratory data points for R(X_H) and [], respectively, taken from Angus & Jansen (1988) and references therein.