code: def hessian(self, coordinates):
          
          # N3 is 3 times the number of atoms.
          N3 = coordinates.size
          # Start with a zero hessian.
          hessian = numpy.zeros((N3,N3), float)
          # Add the contribution of each term.
          for term in self.terms:
              term.add_to_hessian(coordinates, hessian)
          return hessian
docs: Compute the force-field Hessian for the given coordinates.
  
             Argument:
              | ``coordinates`` -- A numpy array with the Cartesian atom
                                   coordinates, with shape (N,3).
  
             Returns:
              | ``hessian`` -- A numpy array with the Hessian, with shape (3*N,
                               3*N).