A-B and the two separated fragments A and B (see Figure ). The electronic energy is scanned in incremental steps while elongating the bond distance between the hydrogen-oxygen bridge (r O••H ). A morse-potential function is then fitted through these data points, with an additional point given at infinite separation with its magnitude being the electronic energy of two isolated water molecules (Figure ). The entropic contributions -T ∆S are modelled by computing the entropy for the equilibrium structure and structures with small elongation of r O••H . Similarly to the electronic fit, the entropic contributions at infinite separation are added to the dataset and a sigmoidal function is fitted (Figure ). By adding up the electronic fit function and the entropic contributions fit function, the authors yield an function, that estimated the Gibbs free energy surface for the water dimer (c).