Predicting Excited-State Absorption Spectra from Non-Aufbau Configurations

Accurately predicting excited-state absorption (ESA) spectra is crucial for interpreting transient absorption spectroscopy (TAS) features, but it remains a challenge for electronic structure theory. Various density- and wave function-based methods can be used to predict ESA spectra, yet they often struggle to simultaneously achieve accuracy, computational efficiency, and molecular-orbital-based chemical intuition. This work presents a balanced approach for predicting ESA spectra (LR-TDA/ΔSCF) by combining the maximum overlap method (MOM) and Δ self-consistent-field (ΔSCF) with the linear-response Tamm-Dancoff approximation (LR-TDA). LR-TDA/ΔSCF captures excited-state orbital relaxation while preserving the computational efficiency and interpretability of standard LR-TDA calculations. Applying LR-TDA/ΔSCF to excited states of interest enables the mapping of TAS features to electronic and/or geometric species. Here, we benchmark LR-TDA/ΔSCF against femto- and nanosecond TAS data for three chromophores: azobenzene, a BODIPY derivative, and a zinc porphyrin complex. The results demonstrate that LR-TDA/ΔSCF can reproduce the experimental ESA spectra with good accuracy, even when neglecting vibronic effects and attempting to describe multiconfigurational excited states with a single Slater determinant. LR-TDA/ΔSCF offers an accurate and cost-effective approach that enables the assignment of TAS spectral features to particular species and transitions, making it a powerful tool for elucidating photochemical and photophysical mechanisms.