To estimate the size of the search problem posed by torsions in SMIRKY, we considered an example reference torsion t78 ("[*:1]-,:[#6X3:2]=[#7X2:3]-[*:4]"), a deceptively simple example where SMIRKY never reaches a 100% partial score. SMIRKY simulations for torsions are initialized with the center two atoms specified and no bond information provided, so the initial torsion relevant to t78 is "[*:1]∼[#6:2]∼[#7:3]∼[*:4]" -that is, a carbon atom connected by any bond to a nitrogen atom. For the purpose of this exercise we will assume we want to generate this exact same SMIRKS; while that is not actually the goal of SMIRKY, it is a helpful example to illustrate the scope of the combinatorial problem. In this case, the probability of adding any one particular atom or bond decorator is approximately 0.01. In t78, there are an additional two atom decorators (X3 on carbon and X2 on nitrogen) along with four extra bond decorators. For each SMIRKY step, one torsion type from the working set is chosen (this is the parent type) and then the child fragment is created by making changes to this parent type (Figure ). In order to generate this particular SMIRKS we also need to include the probability of choosing the correct parent type, which is about 0.02. Even if we assume the order of adding each decorator is unimportant, it will typically take SMIRKY over 1 billion steps to generate this exact SMIRKS pattern.