Quadruplet compositions

To distinguish ligand v. protein atoms, we have classified the protein and ligand C, N, O and S as different atom types. Hydrogen atoms were not considered since, usually, they are not defined explicitly in X-ray structures. Thus, we have defined four atom types for receptor proteins and six atom types for the ligands. In total, there were 554 possible types of interfacial atomic quadruplet compositions, and each of them gave rise to an independent variable... 

Figure 10.5 Calculation of the EnTess descriptors. The same atom type from receptor and ligand is treated differently. In the formulas, m is the mth quadruplet composition type, n represents the number of occurrences of this composition type in a given protein-ligand complex, and j is the vertex index within the quadruplet.

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