CASPT2 studies on the electronic spectra of linear heteroatom-containing carbon chain anions C4O−, C4S− and C4Se−

Structures of linear valence isoelectronic carbon chain anions C4O−, C4S− and C4Se− in their ground states have been investigated by density functional theory (DFT-B3LYP) and coupled cluster with single-double substitution (CCSD) approach. Complete-active-space-self-consistent-field (CASSCF) method has been used for geometry optimization of selected low-lying states. The vertical excitation energies from the ground state to selected low-lying excited states have been carried out with complete-active-space-second-order-perturbation-theory (CASPT2). All of them have a similar set of excited states. In comparison with available experimental observations, the predicted excitation energies for the allowed transitions have an accuracy of no more than 0.12 eV. Moreover, the calculations here confirm that the previous assignments of the observed electronic absorption bands of C4O− and C4S− are reliable. The excitation energies of 22Π←X2Π and 12Σ−←X2Π transitions in C4Se− are predicted to be 2.09 and 3.90 eV, with oscillator strengths 0.1941 and 0.0105, respectively, which may be observed in future experiment. CASPT2 also evaluated the vertical electron detachment energies for C4O−, C4S− and C4Se− to be 3.55, 3.40 and 3.40 eV, respectively.