Ultra-coarse-graining modeling of liquid water

It is a great challenge to develop ultra-coarse-grained models in simulations of biological macromolecules. In this study, the original coarse-graining strategy proposed in our previous work [M. Li and J. Z. H. Zhang, Phys. Chem. Chem. Phys. 23, 8926 (2021)] is first extended to the ultra-coarse-graining (UCG) modeling of liquid water, with the N_C increasing from 4-10 to 20-500. The UCG force field is parameterized by the top-down strategy and subsequently refined on important properties of liquid water by the trial-and-error scheme. The optimal cutoffs for non-bonded interactions in the N_C = 20/100/500 UCG simulations are, respectively, determined on energy convergence. The results show that the average density at 300 K can be accurately reproduced from the well-refined UCG models while it is largely different in describing compressibility, self-diffusion coefficient, etc. The density-temperature relationships predicted by these UCG models are in good agreement with the experiment result. Besides, two polarizable states of the UCG molecules are observed after simulated systems are equilibrated. The ion-water RDFs from the ion-involved N_C = 100 UCG simulation are nearly in accord with the scaled AA ones. Furthermore, the concentration of ions can influence the ratio of two polarizable states in the N_C = 100 simulation. Finally, it is illustrated that the proposed UCG models can accelerate liquid water simulation by 114-135 times, compared with the TIP3P force field. The proposed UCG force field is simple, generic, and transferable, potentially providing valuable information for UCG simulations of large biomolecules.