O-O hemi-bonding in cationic water clusters: Benchmarks from DFT, PNO-LCCSD(T), and FN-DMC

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Quantum Chemistry

Background:

• Water cluster cations ((H2O)n+) are crucial models for ionized aqueous environments.
• Understanding the stability of O-O hemi-bonded motifs in these clusters is challenging.
• Accurate description of relative energetics and dissociation behavior is needed.

Purpose of the Study:

• To systematically benchmark computational methods for studying cationic water clusters (n=2-8).
• To determine the relative energetics of hemi-bonded versus proton-transferred isomers.
• To evaluate O-O bond dissociation energies and complete dissociation energies.

Main Methods:

• High-level coupled cluster calculations (CCSD(T)-F12b/CBS) were used as reference.
• Benchmarking of various density functional theory (DFT) functionals, including hybrid meta-GGA and double hybrids.
• Explicitly correlated PNO-LCCSD(T)-F12 and diffusion Monte Carlo methods were employed for validation.

Main Results:

• Proton-transferred isomers are consistently more stable than hemi-bonded structures.
• O-O bond dissociation energies significantly exceed hydrogen bond strengths, indicating strong hemi-bonding.
• Hybrid meta-GGA and double hybrid functionals show good performance for relative energies, but dissociation energies remain challenging for most DFT approximations.

Conclusions:

• Proton-transferred structures are the preferred isomers in cationic water clusters.
• Strong three-electron hemi-bonding interactions are present in O-O hemi-bonded motifs.
• Advanced computational methods like explicitly correlated coupled cluster and diffusion Monte Carlo are reliable for larger systems.