FlexS is a computer program for predicting ligand superpositions. For a given pair of ligands, FlexS predicts the conformation and orientation of one of the ligands relative to the other one. In FlexS the reference-ligand is assumed to be rigid, thus, it should be given in a conformation which is similar to the bound state. The superposition algorithm in FlexS requires only little manual intervention. Nevertheless, in some cases additional information about the ligands or even the superposition is known. You can integrate this knowledge into the computations with FlexS by carrying out single steps manually. Thus, FlexS is designed for interactive work on ligand pairs as well as for ligand-based virtual screening.