Revisiting the\nThickness of the Air–Water Interface\nfrom Two Extremes of Interface Hydrogen Bond Dynamics

The air–water interface plays a crucial role in\nmany aspects\nof science because of its unique properties, such as a two-dimensional\nhydrogen bond (HB) network and completely different HB dynamics compared\nto bulk water. However, accurately determining the boundary of interfacial\nand bulk water, that is, the thickness of the air–water interface,\nstill challenges experimentalists. Various simulation-based methods\nhave been developed to estimate the thickness, converging on a range\nof approximately 3–10 (Å). In this study, we introduce\na novel approach, grounded in density functional theory-based molecular\ndynamics and deep potential molecular dynamics simulations, to measure\nthe air–water interface thickness, offering a different perspective\nbased on prior research. To capture realistic HB dynamics in the air–water\ninterface, two extreme scenarios of the interface HB dynamics are\nobtained: one underestimates the interface HB dynamics, while the\nother overestimates it. Surprisingly, our results suggest that the\ninterface HB dynamics in both scenarios converges as the thickness\nof the air–water interface increases to 4 (Å). This convergence\npoint, indicative of the realistic interface thickness, is also validated\nby our calculation of anisotropic decay of OH stretch and the free\nOH dynamics at the air–water interface.