hydrogen atom types used by parm99/parm@Frosst are relatively complex because previous work concluded the nonbonded parameters for these atoms need to be different, and thus SMARTS patterns to match these must also be fairly complex. Specifically, these atom types require SMARTS strings extending out far enough to describe multiple atoms in the beta position. If SMARTY did not use SMARTS patterns already generated as parent types, finding these complex patterns would be impossible. Here, the relevant atom types cover hydrogen attached to carbon with zero (HC), one (H1), two (H2) or three (H3) electron withdrawing groups (Figure ). For example, a SMARTS pattern which can recognize H3 (in AlkEthOH as shown in Figure