code: def yield_pair_hessians(self, index1, index2):
          
          d_1 = 1/self.distances[index1, index2]
          d_3 = d_1**3
          if self.charges is not None:
              c1 = self.charges[index1]
              c2 = self.charges[index2]
              yield 2*c1*c2*d_3, np.zeros((3, 3))
          if self.dipoles is not None:
              d_5 = d_1**5
              d_7 = d_1**7
              p1 = self.dipoles[index1]
              p2 = self.dipoles[index2]
              yield 12*d_5*np.dot(p1, p2), np.zeros((3, 3))
              yield -90*d_7, np.outer(p1, p2) + np.outer(p2, p1)
              if self.charges is not None:
                  yield 12*c1*d_5, np.zeros((3, 3))
                  yield 12*c2*d_5, np.zeros((3, 3))
docs: Yields pairs ((s''(r_ij), grad_i (x) grad_i v(bar{r}_ij))