Cooperativity and Frustration Effects (or Lack Thereof) in Polarizable and Non-Polarizable Force Fields

Understanding cooperativity and frustration is crucial for studying biological processes, such as molecular recognition and protein aggregation. Force fields have been extensively utilized to explore cooperativity in the formation of protein secondary structures and self-assembled systems. Multiple studies have demonstrated that polarizable force fields provide more accurate descriptions of this phenomenon compared to fixed-charge pairwise non-polarizable force fields, thanks to the incorporation of polarization effects. In this study, we assess the performance of the AMOEBA polarizable force field and the AMBER and OPLS non-polarizable pairwise force fields in capturing positive and negative cooperativity recently explored in neutral and charged molecular clusters using Density Functional Theory. Our findings show that polarizable and non-polarizable force fields qualitatively reproduce the relative cooperativity observed in electron structure calculations. However, AMBER and OPLS fail in describing absolute cooperativity. In contrast, AMOEBA accounts for absolute cooperativity by considering interactions beyond pairwise interactions. According to the energy decomposition analysis, it is observed that the electrostatic interactions calculated with the AMBER and OPLS force fields seems to play an important and counter-intuitive role in reproducing the adiabatic interaction energies calculated with Density Functional Theory. However, it is important to note that these force fields, due to their nature, do not explicitely incorporate many-body effects, which limits their ability to accurately describe cooperativity. On the other hand, frustration in polarizable and non-polarizable force fields is caused by changes in bond stretching and angle bending terms of the building blocks when they are forming a complex.