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5.2 solvent thf interaction with zno 65Grimme correction [108], is as large as 0.97 eV in nice agreementwith the MPMD result (see above). This large ZnO/THFinteraction turns out to be larger than both P3HT/P3HTcohesive energy (0.1 eV/thiophene) [3] and ZnO/P3HT interaction(0.7 eV/thiophene) (see chapter 3).5.2.2 Interaction between the THF liquid solvent and ZnO surfaceat room temperatureIn this section we consider the interaction between a THFliquid solvent and the ZnO surface at room temperature. Inorder to obtain a realistic model of the liquid solvent, a simplecubic crystal formed by 216 THF molecules is melted athigh temperature. The liquid is then cooled down to roomtemperature and equilibrated in the constant-pressure, constanttemperature(NPT) ensemble at ambient conditions by usinga Nosé-Hoover barostat and thermostat. The equilibriumdensity of the final liquid is found to be 0.879 g/cm 3 ,in agreement with previous theoretical results [106] andclose to the experimental value 0.884 g/cm 3 [109]. A portionof this liquid is cut and merged to ZnO and the resultingsolid-liquid system is equilibrated at room temperaturefor 0.2 ns in a simulation cell as large as 45x65x92Å (see Figure 5.6). After few picoseconds can be observedthe formation of an ordered (and hereafter stable) monolayerof THF molecules wetting the ZnO surface. Most ofthe molecules in the layer are stuck on the substrate as inthe single molecule case, with the oxygen of THF boundto the zinc atom on the surface, suggesting that part of theTHF molecules efficiently bind to ZnO during synthesis insolution.5.2.2.1 ZnO/THF density profileIn order to characterize the interface and its local structure,the simulated system is divided into slices along zdirection, setting z = 0 Å at the ZnO surface. For eachslice is calculated the density ρ, obtaining the density profileρ(z) reported in Figure 5.6. Far from the interface, atz < 0 Å and at z > 5 Å, ρ(z) is constant and similar tothe value of the ZnO crystal (ρ ZnO = 5.6 g/cm 3 ) and thatof the THF liquid (ρ THF ), respectively. The ZnO/THF inter-