code: def running_coordination_number(coordinates_a, coordinates_b, periodic, 
                                  binsize=0.002, cutoff=1.5):
      
      x, y = rdf(coordinates_a,
                 coordinates_b,
                 periodic=periodic,
                 normalize=False,
                 binsize=binsize,
                 cutoff=cutoff)
      y = y.astype('float32') / len(coordinates_a)
      y = np.cumsum(y)
      return x, y
docs: This is the cumulative radial distribution 
      function, also called running coordination number