Enhanced Si–O Bond Breaking in Silica Glass by Water Dimer: A Hybrid Quantum–Classical Simulation Study

A hybrid quantum–classical simulation of a 4,608-atom silica glass is performed at a temperature of 400 K with either a water monomer or dimer inserted in a void. The quantum region that includes the water and the surrounding atoms is treated by the density-functional theory (DFT). During a simulation, the silica glass is gradually compressed or expanded. No Si–O bond breaking occurs with a water monomer until the silica glass collapses. With a water dimer, we find that Si–O bond breaking occurs through three steps in 3 out of 24 compression cases: (i) H-transfer as 2H2O → OH− + H3O+ accompanied by the adsorption of OH− at a strained Si to make it five-coordinated, (ii) breaking of a Si–O bond that originates from the five-coordinated Si, and (iii) H-transfer from H3O+ to the O of the broken Si–O bond. A separate DFT calculation confirms that the barrier energy of the bond breaking with a water dimer under compression is smaller than that with a water monomer and that the barrier energy decreases signific...