docs: def _load_data_index(self, chain1, chain2):
          """
          Load the coordinates of the CA of the common residues.
          """
          parser = PDB.PDBParser(QUIET=True)
  
          structure1 = parser.get_structure(chain1, self.pdb1)
          structure2 = parser.get_structure(chain2, self.pdb2)
  
          residues1 = list(structure1.get_residues())
          residues2 = list(structure2.get_residues())
  
          indexes1 = set(r.id[1] for r in residues1)
          indexes2 = set(r.id[1] for r in residues2)
  
          indexes = indexes1.intersection(indexes2)
          self.indexes = indexes.copy()
          self.N = len(indexes)
  
          coord1 = []
          indexes1 = indexes.copy()
          for r in residues1:
              if r.id[1] in indexes1 and 'CA' in r:
                  coord1.append(np.concatenate((r['CA'].get_coord(), (1,)))[:, None])
                  # Remove from index to avoid repeated residues
                  indexes1.remove(r.id[1])
          coord1 = np.hstack(coord1).astype(DTYPE, copy=False)
  
          coord2 = []
          for r in residues2:
              if r.id[1] in indexes and 'CA' in r:
                  coord2.append(np.concatenate((r['CA'].get_coord(), (1,)))[:, None])
                  indexes.remove(r.id[1])
          coord2 = np.hstack(coord2).astype(DTYPE, copy=False)
  
          self.coord1 = coord1
          self.coord2 = coord2