The first hyperpolarizability of the four trans forms of a dithienylethene indolinooxazolidine biphotochrome in acetonitrile solution has been evaluated by using two solvation models, an explicit and an implicit one. The implicit solvation model is the integral equation formalism of the polarizable continuum model (IEF-PCM), whereas in the explicit-one, the solvent molecules are represented by point charges, of which the positions have been generated by Monte Carlo simulations whereas the solute is treated quantum. Mechanically. At optical frequencies first hyperpolarizabilities calculated with the implicit solvation model are usually larger than those With the multiscale approach, However both approaches predict similar contrasts, indicating that implicit solvation models such 8 IEF-PCM are Well-suited to describe the variations in the NLO responses of molecular switches. In addition, the analysis of :the contrasts of first hyperpolarizabilities shows that the biphotochrome can act as a three-state NLO switch.