def displ_single(self, g_num, at_1, at_2):
        import numpy as np
        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 % self.num_atoms) == (at_2 % self.num_atoms):
            return np.array([0.0, 0.0, 0.0])
        g = self.geom_single(g_num)
        displ = np.array([ g[i + 3*at_2] - g[i + 3*at_1] for i in range(3) ])
        return displ