Molecular Dynamics-Instantaneous Frequencies of Molecules Method to Address the Performance of Classical and Rigid...

The accurate parametrization of molecular force fields (FFs) is crucial for obtaining reliable descriptors from molecular dynamics simulations. Benchmarking FFs requires methods that test directly or indirectly the changes in interaction potential caused by fluctuations in the chemical environment. This work uses the Molecular Dynamics-Instantaneous Frequencies of Molecules (MD-IFM) method as a tool for FF benchmarking, which allow us to compute vibrational spectroscopic observables, including central frequencies (ω₀), frequency fluctuation amplitudes (σ), and spectral diffusion correlation times (τ_c), from MD simulations. We demonstrate the utility of this method by evaluating the vibrational descriptors obtained for common water FFs (TIP3P, SPC/E, TIP4P/Ew, TIP5P, and OPC) against the high-level MB-pol potential, as well as how these descriptors are useful for addressing solvation structure and dynamics. While the classical FFs performed well in predicting vibrational frequencies of water, they systematically underestimated the heterogeneity of the local environment (σ) and show faster spectral diffusion dynamics (lower τ_c values) compared to MB-pol. Analysis suggests that these shortcomings are linked to a simplistic representation of the hydrogen-bonding network. In addition, our results validate MD-IFM as an efficient and robust approach for FF validation.