Figure shows a sample working atom type hierarchy tree (top panel) that was generated during a SMARTY simulation, with levels of the hierarchy represented by different colors. The bottom panel shows which reference atom types are matched by each SMARTS pattern, and the partial score for each match. It is worth noting in this regard that SMARTY's hierarchical approach to sampling dramatically simplifies the complex search problem. For example, in the parm99/parm@Frosst force field for AlkEthOH, there are four atom types for hydrogens bound to carbon, differing in the number of electron withdrawing groups bound to the carbon atom (i.e., in the beta position). As mentioned before, these hydrogens provide a significant challenge to SMARTY. With hierarchical sampling, SMARTY can first discover less specialized SMARTS and then proceed to more complex derivative types further down the tree (Figure ). SMARTY would not be able to find H1 ("[#1$(*-[#6]-[#8])]"), a carbon with one electron withdrawing group, without first discovering HC ("[#1$(*-[#6])]"), because moves are made one atom at a time. This also highlights the importance of the hierarchy when matching; if the more complex child SMARTS patterns were placed above their parents, then all hydrogen atoms would be assigned to the more general "[#1]" type; the more complex pattern is only valuable when it is a child of and placed below the less complex pattern.