Accurate modeling of excitation energies in complex environments is essential for interpreting spectroscopic signatures. A central challenge which is relevant to many systems, is achieving a balanced description of excited states with distinct electronic characters, such as locally excited (LE) and charge transfer (CT) states, while consistently incorporating environmental effects. Here, we compare the linear-response (LR) and state-averaged (SA) formulations of CASSCF when coupled to the polarizable AMOEBA force field. Using the LE and CT excitations of the substrate–cofactor pair in the photoenzyme CvFAP as a test case, we show that LR- and SA-CASSCF offer complementary strengths. LR-CASSCF provides an unbiased mapping of the excitation manifold, whereas SA-CASSCF can refine selected states to obtain a more quantitative description. This combined strategy offers a robust framework for interpreting spectroscopic features of embedded chromophores, especially in systems where CT character and environmental polarization play a decisive role.
Nottoli, T.; Lapi, L.; Londi, G.; Lipparini, F. & Mennucci, B.
Excitation Energies in a Polarizable Environment: A Comparison of State-Averaged and Linear-Response CASSCF/AMOEBA Formulations.
Photochemistry and Spectroscopy 2026, 2 (1), 10
Read the full paper here: https://doi.org/10.53941/ps.2026.100010
