Density functional response theory calculations of three-photon absorption

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Abstract
Three-photon absorption probabilities δ3PA have been calculated through application of a recently derived method for cubic response functions within density functional theory (DFT). Calculations are compared with Hartree–Fock (HF) and with a coupled cluster hierarchy of models in a benchmarking procedure. Except for cases having intermediate states near resonance, density functional theory is demonstrated to be in sufficient agreement with the highly correlated methods in order to qualify for predictions of δ3PA. For the larger systems addressed, a set of acceptor A and donor D substituted π-conjugated systems formed by trans-stilbene and dithienothiophene (DTT), we find noticeable differences in the magnitude of δ3PA between HF and DFT, although similar trends are followed. It is shown that the dipolar structures, TS-AD and DTT-AD, have substantially larger δ3PA than other types of modifications which is in accordance with observations for two-photon absorption. This is the first application of density functional theory to three-photon absorption beyond the use of few-state models.