Revealing the Heterogeneity of Free OH Groups at the Air-Water Interface via Neural Network-Based Simulation

The orientational distribution and dynamic properties of free hydroxyl (OH) groups are crucial for understanding the air-water interface. However, ongoing debates persist regarding their orientation and energy relaxation, primarily because the structural and dynamic heterogeneity of these groups remain poorly understood. Molecular dynamics (MD) simulations have been widely used to study interfacial free OH groups and hydrogen bond networks. Nevertheless, the classical MD approach struggles to accurately describe many-body (MB) interactions in water, while ab initio MD (AIMD) is limited by its high computational cost. Although neural network (NN)-based MD simulations strike a balance between efficiency and accuracy, many existing deep neural network (DNN) models are typically trained on data sets derived from inaccurate density functional theory (DFT) methods, leading to a poor representation of MB interactions in water clusters. In this study, we performed MD simulations using a DNN model trained on the high-precision MB-pol potential (namely, the DNN/MB-pol model). Our simulations identified two stable conformations of free OH groups that were not clearly resolved by conventional DNN models trained on data from the SCAN-based functionals, highlighting the decisive role of MB interactions in shaping the local potential energy landscape of interfacial water. These two stable conformations of free OH groups correspond to a fast-reorienting state (fast free OH) and a slow-reorienting state (slow free OH). Potential of mean force (PMF) calculations further reveal that the orientational dynamics of the two free OH states are closely correlated with the intermolecular O-H···O distance between the free OH group and its nearest water neighbor. Specifically, a shorter intermolecular O-H···O distance facilitates the reorientation of free OH groups. Additionally, we found that the energy barrier between the fast and slow free OH states is influenced by the local arrangement of water molecules surrounding the free OH group and that the transition between the two free OH states is orientation-dependent. To our knowledge, this work reveals for the first time that the fast free OH state serves as a crucial intermediate connecting the free OH and hydrogen-bonded OH states.