As mentioned in the Methods section, each single site Ga 3+ ion in an amorphous silica model is introduced by a substitution technique, resulting in a Si-OH moiety. In total, five Si atoms, as shown in Figure (1), are considered and substituted individually. Since a Si atom binds to four O atoms, there are four possible locations where a proton can be added. Therefore, a total of 20 DFT optimizations are performed to develop the Ga site structures for ethylene oligomerization on Ga/SiO2.