Histone modifications are implicated in epigenetic inheritance and are of central importance in regulating chromatin structure and gene expression. A prototype example is the trimethylation (Me3) of lysine 9 on histone 3 (H3), which is a readout by an aromatic cage of the chromodomain of heterochromatin-associated protein I (HP1) thereby leading to transcriptional repression and heterochromatin formation. Considering that the lysine methylation does not change the charge state of the histone tail and such aromatic-cage mediated recognition of the quaternary ammonium moiety is emerging as the most striking mechanistic commonality for the state-specific recognition of histone lysine methylation, it is of particular interest and importance to understand the physical origin regarding how the aromatic cage distinguishes between the H3K9Me3 mark and its unmethylated counterpart. Here we have simulated relative binding free energies among HP1 chromodomain-H3 tail complexes differing at position 9 of the H3 tail. Our simulated results further confirm the essential role of cation-a interactions for the binding of a methylated H3 tail by an HP1 chromodomain but indicate that the effect from an electrostatic origin is not dominant in distinguishing between the H3K9Me3 mark and its unmethylated counterpart. Meanwhile, our calculated free energy difference between H3-terf-butyl norleucine 9 and H3-methylnorleucine 9 in their binding to the HP1 clearly reveals the importance of the charge independent interactions for the state-specific readout of histone lysine trimethylation marks