Although a slight increase in the percentage of B monomers was observed during the cooling process from 1500 to 300 K, the degree of this increase was only about 5%. This result is associated with the fact that linkage reactions between isolated units-such as the formation of a dimer from two monomers-are rare events. The reaction events during the quenching process were clarified in detail by analysis using transition matrices. The reaction events during the quenching process were counted based on the structural assignment of boron units. Reactive analyses based on MD trajectory data have previously been applied to explore the chemical reactivity and pathways for organic compounds; here, a reaction event for RE borate glass melts was defined as a change in the assigned boron units before and after one AIMD step (1 fs). Boron units missing O corresponding to the transition state, such as B1O2 in Fig. , were additionally found in the melt.