code: def yield_pair_gradients(self, index1, index2):
          
          A = self.As[index1, index2]
          B = self.Bs[index1, index2]
          distance = self.distances[index1, index2]
          yield -B*A*np.exp(-B*distance), np.zeros(3)
docs: Yields pairs ((s'(r_ij), grad_i v(bar{r}_ij))