Starting from the empty Ga site (species 1 in Fig. ), a C-H bond in ethylene is heterolytically cleaved, producing a vinyl group, and the resulting proton (Ha) from the cleavage goes to an oxygen (Oa) and breaks the Ga-O bond. The newly formed Si-OH group becomes distant from the Ga site, with a Ga-O distance of 4.2 Å. The heterolytic cleavage is exothermic, with a free energy change of -1.0 eV and an activation energy of 0.9 eV (step 1 to 2). Before the subsequent ethylene insertion occurs, a local energy minimum is found where ethylene is physisorbed on the Ga site (species 3). The C=C double bond coordinates to the planar, triangle geometry created by the Ga atom and two adjacent oxygen atoms (Ob and Oc). The physisorption is exothermic on the 3CN site (step 2 to 3, -0.2 eV), after which the migratory insertion of another ethylene molecule leads to a Ga-butenyl species with an activation barrier of 1.5 eV (step 3 to ‡ 2 ).