Assessing Intermolecular Interactions in Guest-Free Clathrate Hydrate Systems.

Area of Science:

• Computational chemistry
• Materials science
• Physical chemistry

Background:

• Empty hydrate structures (sI, sII, sH) are low-density ice forms studied experimentally and computationally.
• Their synthesis motivates research into the stability of guest-free clathrate structures.

Purpose of the Study:

• To predict the stability of empty hydrate structures using advanced computational methods.
• To assess the accuracy of various water models and density functional approximations (DFAs) for these systems.
• To establish benchmark interaction energies for clathrate-like water systems.

Main Methods:

• Employed semiempirical and ab initio-based water models alongside dispersion-corrected density functional theory (DFT).
• Utilized converged wave function-based DF-MP2 calculations for reference data.
• Applied extrapolation schemes with large basis sets (triple/quadruple and quadruple/quintuple ζ) and counterpoise corrections for high accuracy.
• Tested eleven water structures from the WATER27 database and clathrate cavities (sI, sII, sH) with varying numbers of water molecules.

Main Results:

• New benchmark interaction energies were reported for dodecahedron, edge/face-sharing, and fused cube water clusters, and sI, sII, sH clathrate cavities.
• Assessed the performance of seven popular water models and nine DFAs (meta-GGAs, hybrids, range-separated) against benchmark data.
• Dispersion corrections were found to reduce interaction energy errors for most DFT functionals.
• Identified general trends for extending the applicability of models to larger systems despite current limitations.

Conclusions:

• Accurate prediction of empty hydrate stability requires large basis sets and counterpoise corrections.
• Dispersion-corrected DFT shows promise for studying clathrate-like systems, though challenges remain.
• The study provides valuable benchmark data and insights into the performance of computational methods for water clusters and hydrates.