# Performance of Density Functional Approximations in Calculations of Electronic Two-Photon Transition Strengths of Fluorescent Dyes

## TL;DR

This study compares the accuracy of three DFT functionals in predicting two-photon absorption strengths of fluorescent dyes against a high-level CC2 method.

## Contribution

The work evaluates B3LYP, PBE0, and CAM-B3LYP for two-photon transition strengths in donor–acceptor dyes using a generalized few-state model.

## Key findings

- All three functionals show significant deviations from reference CC2 results for two-photon transition strengths.
- The discrepancies are most pronounced in molecules with the highest two-photon absorption strength.
- The generalized few-state model helps explain the behavior of the functionals based on electronic structure parameters.

## Abstract

The performance of
B3LYP, PBE0, and CAM-B3LYP functionals in the
prediction of the two-photon transition strengths (for low-energy
transitions) of 18 dipolar donor–acceptor systems containing
a difluoroborate moiety was evaluated against results obtained using
the resolution-of-identity implementation of the coupled-cluster CC2
model. The generalized few-state model approach, in which the two-photon
transition strength is expressed in terms of electronic structure
parameters, i.e., excitation energies, dipole moments, and transition
dipole moments, was applied to gain deeper insight into the behavior
of selected exchange-correlation functionals. The obtained results
show that all three functionals provide two-photon transition strengths
that differ significantly from the reference strengths, especially
in the case of molecules exhibiting the highest 2PA strength.