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