The electronic spectrum of linear and rhombic C4

Large-scale multi-reference configuration interaction calculations are employed in order to calculate the vertical electronic spectrum of C4 in its linear and rhombic geometrical conformations up to 6.5 eV. The recently measured transitions of linear C4 at 0.93 eV (3Πu←X3Σ−g) and at 3.26 eV (3Σ−u←X3Σ−g) are confirmed. The first allowed transition of rhombic C4 is calculated at 2.15 eV (1B1u←X1Ag) and this transition should help to identify rhombic C4 since there are no transitions of linear C4 in this energy region.