Neglecting the hyperfine structure (which is often justified — unless extremely weak electric fields are considered), the polarizability tensor of atoms is isotropic, α α i j ≡ ≡ α α 0 δ δ i j ⟹ ⟹ E ( 2 ) = − − 1 2 α α 0 F 2 . For some molecules this expression is a reasonable approximation, too.