The first hyperpolarizability of nitrobenzene in benzene solutions: investigation of the effects of electron correlation within the sequential QM/MM approach.

The first hyperpolarizability of nitrobenzene in benzene solutions is evaluated by adopting the sequential-Quantum Mechanics/Molecular Mechanics approach at different correlated wavefunction and density functional theory levels of approximation in order to compare these methods for predicting the solvent effects and in particular the effects of nitrobenzene concentration, which modifies the polarization field due to the surrounding. The liquid configurations are generated using Monte Carlo simulations and the surrounding molecules are represented by point charges, defining an electrostatic embedding. At all levels of approximation, the higher the concentration in nitrobenzene, the larger the first hyperpolarizability of the targeted molecule. At optical frequencies (λ = 1064 nm), for the whole range of concentrations, increasing the amount of Hartree-Fock exchange in the exchange-correlation functional leads to the following observations (i) β∥ and βHRS decrease attaining a minimum at the HF level, (ii) the octupolar component to βHRS increases, (iii) the HRS βsolv/βisol ratio increases, and (iv) the EFISHG βsolv/βisol ratio displays a less systematic behavior. Considering the static properties, for which reference CCSD(T) values have been evaluated, M05-2X, LC-BLYP (μ = 0.33), and M11 are the most reliable exchange-correlation functionals for predicting both βHRS and its evolution as a function of the nitrobenzene concentration whereas in the case of β∥, these are M05-2X, LC-BLYP (μ = 0.28 and 0.33), and CAM-B3LYP.