Origin of Structural Variations in Amorphous SiO₂ Generated by Melt-Quench Simulations

Melt-quench simulations are widely used to generate structural models of amorphous silicon dioxide (a-SiO₂), a technologically important material. However, reported results often divergeparticularly regarding the presence of short Si-Si distances near 2.4 Å, typically interpreted as structural defects. Using extensive ab initio molecular dynamics simulations, we demonstrate that these inconsistencies originate from insufficient equilibration of the liquid prior to quenching. We identify two distinct liquid regimesa high-energy state (HES) and a low-energy state (LES)and show that quenching from the HES consistently produces defects such as edge-sharing SiO₄ tetrahedra and 3-fold-coordinated Si atoms, whereas quenching from the LES yields perfectly coordinated, defect-free amorphous structures. The transition between these regimes depends sensitively on temperature and thermostat friction parameters, explaining the variability observed in previous studies. These findings underscore the critical role of liquid equilibration in generating reliable structural models for amorphous materials using the melt-quench method.