Benchmarking Isotropic and Anisotropic Polarization Force Fields against ALMO/DFT for Charge Carrier Stabilization in Organic Semiconductors

Accurate modeling of charge carrier energetics in organic semiconductors is essential for understanding and optimizing their performance in optoelectronic devices. In this work, we employ the absolutely localized molecular orbital (ALMO) method within density functional theory (DFT) as a quantum mechanical benchmark to evaluate the performance of isotropic (ISO) and anisotropic (ANISO) polarization models implemented in the AMOEBA polarizable force field (PFF). By analyzing a set of representative p-type and n-type organic semiconductors in both bulk-like (center-site) and surface-like (edge-site) cluster configurations, we assess the ability of each model to reproduce ALMO-based apparent polarization energies. Our results show that both ISO and ANISO models yield comparable accuracy in centrosymmetric (bulk-like) environments due to the suppression of anisotropic polarization effects. However, the ANISO model significantly outperforms the ISO model in asymmetric configurations, such as at molecular surfaces or heterojunction interfaces, where direction-dependent polarization becomes non-negligible. ALMO-based energy decomposition analysis (ALMO-EDA) reveals that polarization interactions are the dominant contribution to the transport gap in the condensed phase. This finding provides a simplified theoretical framework for estimating the transport gap by considering only polarization effects. Overall, this study establishes the reliability of the ALMO method as a reference for evaluating polarization models and highlights the importance of incorporating anisotropic polarizability in force fields for accurate modeling of charge localization and transport phenomena in organic materials.