As described above, a key point of Fragmenstein is obedience to parent hits. To emphasise the importance of fidelity to position of the initial hits of the initial hits of Mac1 were combined pairwise ignoring positional information in three different approaches. In one the parent hits were merged by maximum common substructure (MCS), in another by BRICS decomposition, and in a third with Fragmenstein but constrained to a single hit. The minimisation of these in place via constraints to both the parent hits did not yield any acceptable poses, whereas the minimisation in place against only the larger hit resulted in a jump to 23% for MCS and 34% for BRICS (Figure ). When Fragmenstein mergers were constrained to a single hit, the acceptance rate increased from 11% to 14%, because several mergers that were irreconcilably strained when constrained against two hits were more relaxed when constrained against a single hit and not obliged to respect the position of the second hit. The number of interactions formed as determined via PLIP reveals a median 0.25 interactions per heavy-atom (HA) for the acceptable two-hit-constrained Fragmenstein mergers and a lower 0.21 interactions/HA for single-hit-constrained Fragmenstein mergers. This is because without the positional constraints the force-field dominates the placement by pushing towards a distant energy minimum. Fragmenstein utilises molecular mechanics but does not find the energy minimum within a box, and instead finds a low energy state around the initial hit. As a consequence, the calculated free energy of binding are sensitive to the number of constraints applied and are not an overly meaningful metric. Unsurprisingly the median ligand efficiency improves from -0.20 kcal/mol/HA for the twohit-constrained mergers to -0.23 kcal/mol/HA for the single-hit-constrained mergers, despite the latter forming less meaningful interactions by not obeying the conformation of the second hit. The pure-MCS mergers constrained to the largest hit had both fewer interactions and worse free energy of binding (median ligand efficiency of -0.14 kcal/HA) due to the more compact nature, making the mergers more likely to fall off an energy cliff. This contrasts with BRICS decomposition where the substructures of the parent hits are joined at the broken bonds therefore respecting the axis of the compounds, even if they may not have been spatially overlapping. In the BRICS approach, the constraints were to a substructure of single hit, so the ligand efficiency is better than Fragmenstein (-0.25 kcal/mol/HA), whereas the median number of interactions was actually lower (0.17 interactions/HA).