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68 interaction between tetrahydrofuran solvent and zinc oxidezyxzxyFigure 5.7.: Structure factor in the x (top) and y (bottom) directionfor the wetting layer C (left) and the liquid THF close tothe surface L ′ (right).creasing distance z is calculated. At infinite distance, thiswork is by definition the adhesion energy γ A/B of the twoparts A e B. In the case where a molecule is cut by theplane, the whole molecule is attributed to the part (A orB) containing its oxygen. The calculated γ A/B is directlyrelated to energies of the generated surfaces (σ A and σ B );in particular, γ A/B = σ A + σ B . We consider three cuts (seeFigure 5.6): (i) L/L, separating two halves of the bulk liquid;(ii) C/L ′ , separating the crystalline layer C from theneighboring liquid THF layer (L ′ ); (iii) Z/C, separating theZnO crystalline surface (Z) from the wetting layer (C).The L/L work of separation (w L/L ) as a function of thedistance is reported as red curve in Figure 5.8 and it isnormalized to the asymptotic value. This work varies untilthe two semi-bulks are interacting. At distances largerthan the interaction range (z 0 ∼ 7 Å) the work reachesthe asymptotic value γ L/L . Because of the statistic distributionof molecules in the liquid phase, γ L/L is found toslightly depend on the position of the cut. For this reasonwe averaged the results over different cuts and we find