Molecular dynamics simulations show that the expansion of silica glass occurs by the presence of the hydroxyl (SiOH) groups present in the glass as opposed to intact water (H2O) molecules, providing an accurate molecular description of the experimentally observed volume changes in silica glass exposed to water. Using a robust and accurate reactive potential, the simulations show that the expansion is caused by the rupture of siloxane (Si–O–Si) linkages in the glass via reactions with water molecules, forming SiOHs. Such reactions remove smaller rings and form larger rings, with a decrease in the overall number of rings smaller than a prescribed large ring size in comparison to dry glasses. This change in ring structure overcomes the inherently stronger hydrogen bonding in the glasses containing SiOH in comparison to the glasses containing predominantly intact H2O molecules. This stronger H‐bonding of the SiOH also causes a shift to lower frequencies in the high‐frequency OH vibrational spectrum for the silanols, as shown in previous ab‐initio calculations. This introduces a question about assuming the lower frequency part of the high‐frequency peak is only due to intact H2O molecules. A slight decrease in volume occurred in the glasses containing the largest concentration of intact H2O molecules. There is no change in the ring size distribution between the H2O glasses and dry glasses. Rather, the slight decrease in volume in the H2O system is caused by a decrease in siloxane bond angles caused by the formation of H‐bonds between the H2O molecules and the glass O in the siloxane cages surrounding the H2O molecules.
This content will become publicly available on February 7, 2025
In this work, we propose a linear machine learning force matching approach that can directly extract pair atomic interactions from ab initio calculations in amorphous structures. The local feature representation is specifically chosen to make the linear weights a force field as a force/potential function of the atom pair distance. Consequently, this set of functions is the closest representation of the ab initio forces, given the two-body approximation and finite scanning in the configurational space. We validate this approach in amorphous silica. Potentials in the new force field (consisting of tabulated Si–Si, Si–O, and O–O potentials) are significantly different than existing potentials that are commonly used for silica, even though all of them produce the tetrahedral network structure and roughly similar glass properties. This suggests that the commonly used classical force fields do not offer fundamentally accurate representations of the atomic interaction in silica. The new force field furthermore produces a lower glass transition temperature (Tg ∼ 1800 K) and a positive liquid thermal expansion coefficient, suggesting the extraordinarily high Tg and negative liquid thermal expansion of simulated silica could be artifacts of previously developed classical potentials. Overall, the proposed approach provides a fundamental yet intuitive way to evaluate two-body potentials against ab initio calculations, thereby offering an efficient way to guide the development of classical force fields.
more » « less- Award ID(s):
- 2309000
- NSF-PAR ID:
- 10503145
- Publisher / Repository:
- AIP Publishing
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 160
- Issue:
- 5
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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