Toward assessment of density functionals for vibronic coupling in two‐photon absorption: A case study of 4‐nitroaniline

In this study, we predict vibronic two‐photon absorption (TPA) spectra for 4‐nitroaniline in vacuo. The simulations are performed using density functional theory and the approximate second‐order coupled‐cluster singles and doubles model CC2. Thereby we also demonstrate the possibility of simulations of vibronic TPA spectra with ab initio wavefunction methods that include electron correlation for medium‐sized systems. A special focus is put on the geometric derivatives of the second‐order transition moment and the dipole moment difference between the charge‐transfer excited state and the ground state. The results of CC2 calculations bring new insight into the vibronic coupling mechanism in TPA spectra of 4‐nitroniline and demonstrate that the mixed term is quite large and that it also exhibits a negative interference with the Franck‐Condon contribution. © 2015 Wiley Periodicals, Inc.