Fifty Shades of Water: Benchmarking DFT Functionals against Experimental Data for Ionic Crystalline Hydrates.

Abstract

We propose that crystalline ionic hydrates constitute a valuable resource for benchmarking theoretical methods for aqueous ionic systems. Many such structures are known from the experimental literature, and they contain a large variety of water-water and ion-water structural motifs. Here we have collected a data set (CRYSTALWATER50) of 50 structurally unique "in-crystal" water molecules, involved in close to 100 nonequivalent O-H···O hydrogen bonds. Adozen well-known DFT functionals were benchmarked with respect to their ability to describe these experimental structures and their OH vibrational frequencies. We find that the PBE, RPBE-D3, and optPBE-vdW methods give the best H-bond distancesand that anharmonic OH frequencies generated from B3LYP//optPBE-vdW energy scans outperform the other methods, i.e.,here we performedB3LYP energy scans along the OH stretching coordinate whilethe rest of the structurewas kept fixed at theoptPBE-vdW-optimized positions.