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.
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Excitation Energies in a Polarizable Environment: A Comparison of State-Averaged and Linear-Response CASSCF/AMOEBA Formulations
Author Information
Received: 10 Dec 2025 | Revised: 09 Jan 2026 | Accepted: 13 Jan 2026 | Published: 05 Mar 2026
Abstract
Keywords
CASSCF | linear response | state averaged | AMOEBA | polarizable
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