Figure illustrates the steps of the reductive decomposition of EC. Step 1 involves the strong binding of the EC molecule 1 with a Li + ion to form the ion-pair intermediate 2.This ion-pair intermediate 2 is reduced in step 2 to form the neutral adduct 3; the energy change for this step is large and negative due to the strong electron affinity induced by the Li + ion (in the absence of Li + , addition of an electron to gas phase EC requires about +8 kcal/mol). The adduct 3 subsequently undergoes homolytic ring cleavage, passing through the transition state 4 in step 3 to form the intermediate radical 5 in step 4. A nonplanar pathway in steps 2 and 3 is also possible, but we do not consider it here. Once the radical 5 is formed, there are several possible termination pathways. Here, we study two such pathways, as illustrated in step 5 and 6, which correspond to pathways C and B, respectively, in Ref. 25. The first is a reductive decomposition to form lithium carbonate anion 6 and ethylene 7. The second is a dimerization of the radical 5 to form ethylene 7 and lithium ethylene dicarbonate 8. We note that both lithium carbonate and lithium ethylene dicarbonate have been implicated as major components of the SEI, but there are some conflicting observations. Using the electronic structure methods described in Sec. 2, we calculated the electronic energy differences of all six steps. A T1 diagnostic test on CC calculations confirms that all species are qualitatively single-reference, supporting the application of DFT, CC, and AFQMC with a single determinant trial. Using CCSD(T) as the reference, we then calculate the mean absolute deviation (MAD) over these six steps for each method considered. The detailed reaction energy profile predicted by PBE-D3, CAM-B3LYP-D3, and CCSD(T) is shown in Figure and the MAD from all methods is presented in Figure .