code: def yield_pair_energies(self, index1, index2):
          
          A = self.As[index1, index2]
          B = self.Bs[index1, index2]
          distance = self.distances[index1, index2]
          yield A*np.exp(-B*distance), 1
docs: Yields pairs ((s(r_ij), v(bar{r}_ij))