Previous experimental and theoretical results show that the (2√2x√2)R45° missing-row reconstruction is a stable intermediate state during the early stages of Cu(100) oxidation. When the oxygen coverage on a Cu(100) surface reaches 0.5 monolayers [the c(2x2) phase], the surface structure transforms into the missing-row reconstruction by the release of every fourth row of copper atoms from the top copper layer. The released copper atoms are assumed to then diffuse away. The specific mechanisms and energetics of this transition are not yet fully understood. To investigate this transition, we use density functional theory calculations to predict potential copper-releasing pathways and their energy barriers using the climbing image nudged elastic band method. In the p(2√2x√2) unit-cell, there are two potential copper releasing pathways. For each pathway, two energy barriers are predicted because there is an intermediate state between the c(2x2) phase and the missing-row reconstruction. The energy barriers are 1.61 eV and 1.04 eV for the first pathway and 2.19 eV and 0.38 eV for the second pathway. To assess system size effects and alternative pathways, we also investigate the p(2√2x2√2) and p(4x4) unit-cells. In applying the copper releasing pathways analyzed for the p(2√2x√2) unit-cell to the larger unit-cells, there will be multiple ways to arrive at the final state. For example, in the p(2√2x2√2) unit-cell, two copper atoms should be released to form a complete missing-row. There are two ways for the missing-row formation: moving the copper atoms one by one or moving two copper atoms together. We expect that the copper releasing pathways and energy barriers will vary with the nature of the releasing copper movements.