We studied five alkaline earth dicarbide systems MAEC2 (where M-AE =   Be-Ba) by using ab initio random structure search. For BeC2 and MgC2,   the lowest energy and dynamically stable configuration consists of five-membered carbon rings connected to each other via an individual  arbon atom, stabilized through the donation of electrons from the   surrounding alkaline earth ions. For CaC2, SrC2, and BaC2, our study   shows that the chain crystal structure is more stable than the   predicted structure due to strains induced by the increasing size of   alkaline earth metal ions. The reaction energies of the typical synthesis pathway are comparable to those calculated for the experimental reaction of the known chain-type structure. Finally, the proposed structure should be optically distinguishable due to a   significantly narrower band gap.