In our model the introduction of a phantom wall and the thermodynamic integration step are unnecessary since the wall is structureless, there are no internal stresses, and the wall forces are always normal to the surface. As a result, we can show that Eq. ( ) can also be used for the wall surface energies, provided an augmented definition ⟨P ′ zz ⟩ is used which includes the wall forces. The details are given in the Appendix.