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Motivation:The accurate description of interfaces is needed to identify which residues interact with another molecule or macromolecule. In addition, a data structure is required to compare interfaces within or between families of protein-protein or protein-ligands complexes. In order to avoid many unwanted comparisons, we looked for a parameter free computation of interfaces. This need appeared at the occasion of bioinformatics studies by our research team focusing on HIV-2 protease (PR2) resistance to its inhibitors. Results: We designed the PPIC software (Protein Protein Interface Computation). It offers three methods of computation of interfaces: (1) our original parameter free method, (2) the Voronoi tessellation approach, and (3) the cutoff distance method. For the latter, we suggest on the basis of 1050 dimers protein-protein interfaces that the optimal cutoff distance is 3.7Å, or 3.6Å for a set of 18 PR2-ligand interfaces. We found at most 17 contact residues with PR2 ligands. Availability: Free binaries and documentation are available through a software repository located at