Recently, two of us have demonstrated using the second quantization approach that unlinked terms appearing in the numerator cancel out with the ⟨P ⟩ terms from the denominator (which is apparent after multiplying both sides of Eq. ( ) by 1 + ⟨P ⟩, see also Refs. 55,57). Such a trick also allows the inclusion of P 2n operators of any order, allowing to go beyond the S 2 approximation. Thus, the final formula for ∆X