code: def get_hessian(self):
          
          force_const = self.fields.get("Cartesian Force Constants")
          if force_const is None:
              return None
          N = len(self.molecule.numbers)
          result = np.zeros((3*N, 3*N), float)
          counter = 0
          for row in range(3*N):
              result[row, :row+1] = force_const[counter:counter+row+1]
              result[:row+1, row] = force_const[counter:counter+row+1]
              counter += row + 1
          return result
docs: Return the hessian