Theoretical investigation of the static (dynamic) polarizability and second hyperpolarizability of DAAD quadrupolar push–pull molecules. A comparison among HF (TD-HF), DFT (TD-B3LYP), and MP2 (TD-MP2) methods

Calculations at HF, DFT (SVWN, PBE, B3LYP, PBE0, BHHLYP and CAM-B3LYP functionals) and MP2 level were carried out with 6-31+G(d,p) basis set for estimate the polarizability (α) and the second hyperpolarizability (γ) of DAAD quadrupolar push–pull molecules. Our results show that DFT schemes overestimate α values relative to MP2 ones, being SVWN and PBE those functionals that give the most important deviation, whereas that HF and MP2 results are comparable between them. Moreover, the electron correlation effect is negative at MP2 level. For the static second hyperpolarizability, a significant agreement was obtained between CAM-B3LYP and MP2 methods. In relation to the frequency-dependence, only the dynamic second hyperpolarizability, calculated from TD-HF, TD-B3LYP, and TD-MP2 methods, revealed a large dependence on λ. When 〈γ〉 (〈α〉) are correlated to π∗/π (the ratio between the occupation number of the antibonding orbital, π∗, and the bonding orbital, π) one significant exponential relationship was obtained, similar those obtain previously using the bond length alternation (BLA) parameter. Finally, our results demonstrate that DAAD molecules are more effective for third-order NLO applications than the corresponding dipolar molecules.