PNNL

Abstract

We present a new discrete model for the combinatorial optimization of the various isomers of polyhedral water clusters (PWCs). The “Strong-Weak-Effective-Bond” (SWEB) model is based on the concept of “strong” / “weak” nearest neighbor hydrogen bonds depending on the (trans/cis) orientation of dimer-like arrangements and the connectivity of the respective pairs to neighbors. We identify 5 types of hydrogen bonds on the surface of PWCs based on the trans/cis orientation of the respective pairs and the number and position of their dangling OH stretches. We furthermore define an effective pair interaction energy for each different type of hydrogen bond that incorporates pair interactions between nearest-, second-, and third-neighbors. The analysis of the effective pair energies of the different types of hydrogen bonds suggests the preferential energetic stabilization of a class of isomers that have the maximum number of a hydrogen bond type, defined as (t1d), with the following characteristics: it corresponds to a trans nearest-neighbor orientation and has 1 dangling bond which resides on the donor molecule. For the pentagonal dodecahedron cluster of (H2O)20 there are 64 isomers (out of a total of 30,026 possible) with a maximum number of 7 (t1d)-type and 803 isomers with 6 (t1d)-type hydrogen bonds. The screening of these two lowest energy classes of isomers predicted by the SWEB discrete model with the TIP4P and TTM2-F interaction potentials suggests the grouping of the isomer energies into distinct, non-overlapping regions. Further quantitative electronic structure DFT and MP2 calculations of candidates within the lowest class of 64 isomers predicted with the SWEB model identifies a new global minimum for the dodecahedron family of (H2O)20, which has been previously missed using other sampling methods.