def yield_pair_gradients(self, index1, index2):
strength = self.strengths[index1, index2]<EOL>distance = self.distances[index1, index2]<EOL>yield -<NUM_LIT:6>*strength*distance**(-<NUM_LIT:7>), np.zeros(<NUM_LIT:3>)<EOL>
Yields pairs ((s'(r_ij), grad_i v(bar{r}_ij))