Possible low-energy isomers of OHn(H2O)4 (n = 0, ±1) clusters via the particle swarm optimization algorithm: An ab initio study

Abstract

Abstract The possible low-energy structures of the anionic, neutral, and cationic water clusters OHn(H2O)4 (n\u202f=\u202f0, ±1) were investigated using the particle swarm optimization algorithm together with ab initio calculations. Geometry optimization and vibrational analysis for possible low-lying water clusters were conducted at the MP2/aug-cc-pVDZ level and further verified at the CCSD(T)/aug-cc-pVTZ level. The results revealed obvious structural differences between the OH+(H2O)4, OH(H2O)4, and OH−(H2O)4 clusters. We found that correcting for the zero-point vibrational energies and basis set superposition error exerted a substantial influence on the order of relative energies for the OH−(H2O)4 and OH+(H2O)4 clusters. Topological analysis revealed that both the number and strength of hydrogen bonds were greater for the OH−(H2O)4 cluster compared with the OH+(H2O)4 and OH(H2O)4 clusters. The highest occupied molecular orbitals (HOMOs) of the anionic OH−(H2O)4 cluster were mainly located on the OH− ion and the HOMO-1s and HOMO-2s levels were mainly located on the H2O molecules, whereas in the neutral OH(H2O)4 and cationic OH+(H2O)4 clusters, the singly occupied molecular orbitals mainly originated from the water molecules.