The diffusion of water molecules and clusters across the surfaces of materials is important to a wide range of processes. Interestingly, experiments have shown that on certain substrates, water dimers can diffuse more rapidly than water monomers. Whilst explanations for anomalously fast diffusion have been presented for specific systems, the general underlying physical principles are not yet established. We investigate this through a systematic ab initio study of water monomer and dimer diffusion on a range of surfaces. Calculations reveal different mechanisms for fast water dimer diffusion, which is found to be more widespread than previously anticipated. The key factors affecting diffusion are the balance of water-water versus water-surface bonding and the ease with which hydrogen-bond exchange can occur (either through a classical over-the-barrier process or through quantum-mechanical tunnelling). We anticipate that the insights gained will be useful for understanding future experiments on the diffusion and clustering of hydrogen-bonded adsorbates.

水分子和水团在材料表面上的扩散对许多过程都很重要。有趣的是，实验表明，在某些基材上，水二聚体比水单体扩散得更快。虽然已经针对特定系统提出了异常快速扩散的解释，但尚未建立一般的基本物理原理。我们通过对水单体和二聚体在一系列表面上的扩散进行系统的从头算研究来研究这一点。计算揭示了快速水二聚体扩散的不同机制，发现其比先前预期的更广泛。影响扩散的关键因素是水-水与水-表面键合的平衡以及氢键交换发生的容易程度（通过经典的越障过程或通过量子力学隧道效应）。我们预计所获得的见解将有助于理解未来关于氢键吸附物的扩散和聚集的实验。