Ab initio study of Kubas-type dihydrogen fixation onto d-orbital states of Ca adatoms

► Ca's 3d orbitals can be utilized for H2 fixation through the Kubas interaction. ► On defective graphenes, the adsorbed Ca sites stabilize the 3d orbitals. ► DFT leads to overestimation of the Kubas interaction between H2 and the 4s state.