In contrast, determination of the surface energy for a fluid against a wall can be fraught with difficulties due to the presence of internal stresses in the solid substrate. To overcome these, Leroy and coworkers devised a 'phantom wall' method to compute the surface energy. By incrementally 'lifting' the phantom wall off the real substrate, one can compute the difference in surface energy between the real wall and the phantom wall by thermodynamic integration. This is augmented by a direct calculation of the phantom wall surface energy once the real wall is sufficiently distant to avoid perturbing the fluid.