code: def dist_single(self, g_num, at_1, at_2):
          import numpy as np
          from scipy import linalg as spla
          from .utils import safe_cast as scast
          if not (-self.num_atoms <= at_1 < self.num_atoms):
              raise IndexError("Invalid index for 'at_1' ({0})".format(at_1))
          if not (-self.num_atoms <= at_2 < self.num_atoms):
              raise IndexError("Invalid index for 'at_2' ({0})".format(at_2))
          at_1 = scast(np.floor(at_1), np.int_)
          at_2 = scast(np.floor(at_2), np.int_)
          if at_1 == at_2:
              dist = 0.0
          else:
              dist = scast(
                      spla.norm(self.displ_single(g_num, at_1, at_2)),
                              np.float_)
          return dist