PNNL

Abstract

We rely on a total of 23 (cluster size, 8 structural, and 14 connectivity) descriptors to investigate structural patterns and connectivity motifs associated with water cluster aggregation. In addition to the cluster size n (number of molecules), the 11 structural descriptors can be further categorized into (i) 1-body (intramolecular): covalent OH bond length (rOH) and HOH bond angle (?HOH), (ii) 2-body: OO distance (rOO), OHO angle (?OHO), and HOOX dihedral angle (?HOOX), where X lies on the bisector of the HOH angle, (iii) 3-body: OOO angle (?OOO) and (iv) many-body: modified tetrahedral order parameter (q) to account for 2-, 3-, 4-, 5-coordinated molecules (qm, m= 2, 3, 4, 5) and radius of gyration (Rg). The 14 connectivity descriptors are all many-body in nature and consist of the AD, AAD, ADD, AADD, AAAD, AAADD adjacencies (number of hydrogen bonds accepted (A) and donated (D) by each water molecule), Wiener index, Average Shortest Path Length (ASPL), hydrogen bond saturation (% HB) and number of non-short-circuited 3-, 4-, 5-, 6- and 7-cycles. We mined a previously reported database of 4,948,959 water cluster minima for (H2O)n, n= 3 – 25 to analyze the evolution and correlation of these descriptors for the clusters within 5 kcal/mol of the putative minima. It was found that rOH and %HB correlated strongly with cluster size n, which was identified as the strongest predictor of energetic stability. Marked changes in the adjacencies and cycle count were observed, lending insight into the changes in the hydrogen bond network upon aggregation. A Principal Component Analysis (PCA) was employed to identify descriptor dependencies and group clusters into specific structural patterns across different cluster sizes. The results of this study inform our understanding of how water clusters evolve in size and what are appropriate descriptors of their structural and connectivity patterns with respect to system size, stability, and similarity. The approach described in this study is general and can be easily extended to other hydrogen bonded systems.