Ab Initio Investigation of Gas Hydrates: Structural, Energetic, and Electronic Effects of Guest Molecule Inclusion

This work presents ab initio simulations based on Quantum Density Functional Theory (q-DFT) of the type sII gas hydrate unit cell, considering both the empty structure and configurations filled with propane molecules. All calculations were performed using the VASP (Vienna Ab initio Simulation Package), adopting three-dimensional periodic boundary conditions and treating the sII hydrate as an infinite solid, an approach appropriate for clathrate hydrate systems. Structural and energetic properties were evaluated from electronic ground-state calculations, with a focus on the structural and electronic stability of the system. The study followed a structural optimization strategy employing the conjugate-gradient algorithm (IBRION = 2), with simultaneous relaxation of internal atomic coordinates and the crystal lattice degrees of freedom (ISIF = 7).

The objective was to investigate the influence of guest molecule inclusion on the structural, energetic, and electronic properties of the hydrate. The PBE and RPBE exchange–correlation functionals were tested, including D3-type van der Waals corrections, with consistent comparisons to reference systems such as liquid water and ice phases. The results indicate that propane inclusion significantly modifies the electronic and energetic structure of the system, leading to enhanced structural stability of the filled hydrate relative to the empty unit cell.

Preliminary results show that the RPBE/D3 functional provides an adequate description of both the reference systems and the sII hydrate, highlighting the fundamental role of guest molecules in stabilizing the crystalline framework. As a perspective, future studies will investigate the effects of lattice compressibility under high-pressure conditions.