A theoretical study of CaC3 species has been carried out. As a general trend, triplet states are found to be more stable than singlet ones for all isomers. The predicted global minimum is a rhomboidal species, 4t(B-3(1)), whereas another rhomboidal four-membered ring, 3t((3)A(1)), is predicted to lie quite close in energy (about 8.2 kcal/mol higher in energy at the G2(P) level). The corresponding singlet states, 3s((1)A(1)) and 4s((1)A(1)), lie about 11.5 and 13 kcal/mol, respectively, above the predicted global minimum. Therefore our calculations suggest that several species could be accessible to experimental detection. (C) 2002 Elsevier Science B.V. All rights reserved.