Green’s functions of the electrons and $\Sigma_{\alpha}^{>,<} = \Sigma_{\alpha,L}^{>,<} + \Sigma_{\alpha,R}^{>,<}$ are the total lead self energies. They are functions of energy $E$. This formula is exact provided that the electron Green’s function is obtained exactly. However, such a goal for the Coulomb system is not attainable. Thus, we use the lowest order expansion approximation in terms of the Coulomb interaction. Such approximation preserves energy conservation exactly. Since the two layers are not coupled directly, the Green’s functions for the electrons and self energies are block diagonal, and the Meir-Wingreen formula needs only the block $\alpha$. We focus on layer 1, and Green’s function $G_{1}^>$, can be expressed by the Keldysh equation as $$G_1^> = \bigl[ G^r( \Sigma_1^> +\Sigma_2^> + \Sigma_n^> ) G^a \bigr]_{11}.$$ Here $\Sigma_{1,2}^>$ are the lead self energies, and $\Sigma_n^>$ is the Fock term of Coulomb interaction, all of them block diagonal. For the 11-subblock, $\Sigma_2$ is 0. Putting this result into the Meir-Wingreen formula, noting that ${\rm Tr}( G_1^> \Sigma_1^< - G_1^< \Sigma_1^>)=0$ as a consequence of charge and energy conservation when Coulomb interaction is turned off, we obtain $$I_1 = \int_{-\infty}^{+\infty} \frac{dE}{2\pi \hbar} E\, {\rm Tr} \bigl[ G^r_1 \Sigma_n^> G_1^a 
\Sigma_1^< - ({\rm swap\ } 