Theoretical study on the second hyperpolarizabilities of tetrathiafulvalene (TTF) and tetrathiapentalene (TTP) using highly correlated ab initio MO and the density functional theory methods

We investigate the static second hyperpolarizabilities (γ) of tetrathiafulvalene (TTF) and tetrathiapentalene (TTP) using the ab initio molecular orbital (MO) method. The cationic radical states of these systems (TTF⁺{radical dot} and TTP⁺{radical dot}) are expected to have large negative γ values, which are rare in organic systems, based on our classification rule of γ. It turns out that at the higher-order electron correlation level TTP⁺{radical dot} gives a large negative γ value though TTF⁺{radical dot} gives a positive γ. We also investigate the applicability of the density functional theory (DFT) methods to the calculation of the γ values for these systems. By tuning the mixing parameter of DFT/HF exchange term, a DFT method turns out to semiquantitatively reproduce the γ values of TTP and TTP⁺{radical dot} at the higher-order electron correlation method, i.e., CCSD(T), while fail in reproducing the γ value of TTF⁺{radical dot}. This suggests the necessity of further improvement in correlation functional for obtaining reliable γ values of charged radical states.