<p>Calcite has recently attracted extensive research interest in fields ranging from geoscience to carbon dioxide removal. Although much effort has been made to study the (2 × 1) reconstruction of the most stable (104) surface, the origin of this reconstruction remains unclear. Here, we carefully investigated the atomic and electronic structures of calcite (104) via density functional theory methods with van der Waals corrections. The results unambiguously show that the driving force for this reconstruction is the intrinsic demands of surface atoms to increase the coordination numbers. Upon reconstruction, calcite (104) forms four additional Ca‒O bonds per (2 × 1) unit cell. In addition, the phonon spectra indicate that both the unreconstructed and reconstructed surfaces are dynamically stable. Finally, by applying the climbing image nudged elastic band method, an energy barrier is predicted during the reconstruction. This work provides a full picture of the formation of calcite (104)-(2 × 1) reconstruction and can greatly advance the understanding of surface science for calcite.</p>