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The first hyperpolarizability ofthe four trans forms of a dithienyletheneindolinooxazolidine biphotochrome in acetonitrile solution has beenevaluated by using two solvation models, an explicit and an implicitone. The implicit solvation model is the integral equation formalismof the polarizable continuum model (IEF-PCM), whereas in the explicitone, the solvent molecules are represented by point charges, of whichthe positions have been generated by Monte Carlo simulations whereasthe solute is treated quantum mechanically. At optical frequencies,first hyperpolarizabilities calculated with the implicit solvationmodel are usually larger than those obtained with the multiscale approach.However, both approaches predict similar contrasts, indicating thatimplicit solvation models such as IEF-PCM are well-suited to describethe variations in the NLO responses of molecular switches. In addition,the analysis of the contrasts of first hyperpolarizabilities showsthat the biphotochrome can act as a three-state NLO switch. [ABSTRACT FROM AUTHOR]