One fundamental measure of the interactions responsible for the DCA adsorption process is the potential of mean force (PMF), which describes the free energy change as a function of the distance between the DCA head-group and the deposit. The adsorption free energy is taken as the global minimum value of the PMF, quantifying the affinity of the surfactant for a given surface with respect to staying in the bulk of the solvent. For a reliable PMF production, one must sample distributions favouring regions of phase space that would be infrequently visited owing to the force field interactions, 51 e.g. using ABF. The concept of the collective variable (colvar ), ξ, lies at the core of these methodologies. For the particular problem of adsorption, the most physically meaningful colvar choice is the z-projected distance between the binding groups and the deposit. This is because lateral movements along the x-and y-directions do not significantly alter the free energy as long as the adsorbate remains within the binding sites, and so the Euclidean distance might constitute a suboptimal colvar. Besides accounting for the physical nature of the problem at stake, it is also important to choose the collective variable so that the biasing force is not heavily affected by stochastic noise. Therefore, ξ involves all of the non-fixed atoms in the deposit and all the atoms in the DCA head-group, to cancel out the noisy force terms from the rapidly oscillating bonded interactions. In this way, the variance of the instantaneous force estimator is lowered, improving the ABF convergence.