Pressure-Induced Amorphization of Methane Hydrate: An Ab Initio Molecular Dynamics Study of Guest Molecule Solvation

, the peak position exhibits clear positive correlation with the number of proximate water molecules, whereas this correlation is negligible when the cage structure is maintained at lower densities. Analysis of metastable amorphous states at low densities demonstrates that competing effects govern the C-H stretching vibrational frequency: the approach of first-nearest-neighbor water molecules (blue-shifting) and the weakening of hydrogen bond rigidity between surrounding water molecules (red-shifting). This solvation-state-based analysis represents a key advantage of MD simulations and reveals that focusing on the number of particularly close water molecules is essential for understanding the molecular picture of PIA.