It follows that a large positive Z-score for a building block indicates its statistical over-expression in that subset of molecules relative to the overall screening library. Thus, Z-scores can be used to identify (and quantify the extent of correlation of) the most important building blocks for a catalyst with the least activation barriers for a reaction. Similarly, a large negative Z-score implies a statistical under-expression (or inverse correlation), while Z-scores close to zero imply no statistical correlation of that descriptor to the target property.