Instantaneous energy transfer of water: a comparison between ab initio and classical molecular dynamics

Water is fundamental to a range of natural phenomena and is equally crucial in various engineering applications that demand efficient energy utilization, such as energy conversion with chemical reactions. However, the microscopic mechanisms of energy transfer in water remain unclear. This study investigated the instantaneous energy transfer (IET), formulated using forces obtained from ab initio molecular dynamics (AIMD) with a comprehensive analysis of its correlation with intermolecular and intramolecular structures in liquid water. By comparing our findings with those obtained from classical molecular dynamics (CMD), we examined the validity of the AIMD-based method and assessed the performance of the TIP4P/2005f water model. The results of the O–O, O–H, and H–H radial distribution functions (RDFs) indicated that the strongly constrained and appropriately normed (SCAN) functional in AIMD provided a more accurate representation of experimental values compared with the PBE-D3 functional. On the other hand, although the TIP4P/2005f model in CMD accurately reproduced some structural features, the classical force field exhibited some limitations, particularly in reproducing the height and width of the first peak in the O–O RDF. Moreover, we identified correlations between the IET and distance from the target oxygen atom to its nearest oxygen or hydrogen atoms, revealing that the characteristics of IET depend on this distance. Specifically, the mean IET: IET efficiency (IETE) was higher at shorter interatomic distances, indicating that both instantaneous intermolecular and intramolecular structures determine the IETE. It was also shown that variations in OH bond length significantly contributed to IETE. Additionally, our findings revealed that AIMD utilizing the SCAN functional shows higher IETE compared with that of CMD employing the TIP4P/2005f model. Through this study, the proposed method to evaluate IET has been validated, which will give fundamentals to understand transport phenomena in condensed phase in the framework of AIMD.