Abstract
We investigate the accuracy of different formalisms within density functional theory in prediction of two-photon absorption (2PA) spectra for substituted oligophenylvinylenes compared to the experimental measurements. The quadratic response methods are compared with the recently proposed a posteriori Tamm–Dancoff approximation (ATDA) and previously published third-order coupled electronic oscillator results. Quadratic response is found to overestimate the cross sections in all cases. We trace the reasons to unreliable excited state description above the ionization threshold. In addition, quadratic response lacks the double excitations so that their contributions to the 2PA spectra are redistributed over the nearest single character excitations. This distorts the individual contributions to the 2PA response and affects the overall picture. For this reason, we do not recommend quadratic response for the essential state analysis, while ATDA can be used both for the 2PA predictions and the structure/property ...
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