Adsorption of biomolecules on surfaces is a perennial and general challenge relevant to many fields in biotechnology. A change of the Helmholtz free energy ΔA takes place when a molecule becomes adsorbed out of a bulk solution. The purpose of our investigations is to explore routes for the calculation of ΔA by molecular simulations. ΔA can be obtained both by integration over the mean force on a molecule and via the local density. It turns out that the route via the potential of mean force prevails over the latter due to better consistency. In this work we present results for systems of 1-centre and 2-centre Lennard-Jones mixtures at a 9/3 Lennard-Jones wall.