code: private int nAdjacentCentres(int i) {
          int n = 0;
          for (IAtom atom : tetrahedralElements[i].getLigands())
              if (tetrahedralElements[atomToIndex.get(atom)] != null) n++;
          return n;
      }
docs: Obtain the number of centres adjacent to the atom at the index, i.
  
  @param i atom index
  @return number of adjacent centres