The effect of polarizable environment on two-photon absorption cross sections characterized by the equation-of-motion coupled-cluster singles and doubles method combined with the effective fragment potential approach.

We report an extension of a hybrid polarizable embedding method incorporating solvent effects in the calculations of two-photon absorption (2PA) cross sections. We employ the equation-of-motion coupled-cluster singles and doubles method for excitation energies (EOM-EE-CCSD) for the quantum region and the effective fragment potential (EFP) method for the classical region. We also introduce a rigorous metric based on 2PA transition densities for assessing the domain of applicability of QM/MM (quantum mechanics/molecular mechanics) schemes for calculating 2PA cross sections. We investigate the impact of the environment on the 2PA cross sections of low-lying transitions in microhydrated clusters of para-nitroaniline, thymine, and the deprotonated anionic chromophore of photoactive yellow protein (PYPb). We assess the performance of EOM-EE-CCSD/EFP by comparing the 2PA cross sections against full QM calculations as well as against the non-polarizable QM/MM electrostatic embedding approach. We demonstrate that the performance of QM/EFP improves when few explicit solvent molecules are included in the QM subsystem. We correlate the errors in the 2PA cross sections with the errors in the key electronic properties-identified by the analysis of 2PA natural transition orbitals and 2PA transition densities-such as excitation energies, transition moments, and dipole-moment differences between the initial and final states. Finally, using aqueous PYPb, we investigate the convergence of 2PA cross sections to bulk values.