The ground state and low-lying excited states of CCCN radical and its ions: a CASSCF/CASPT2 study

The ground state and low-lying excited states of the CCCN radical and its ions have been investigated systematically using the complete active space self-consistent field (CASSCF) and multi-configuration second-order perturbation theory (CASPT2) methods in conjunction with the ANO-RCC-TZP basis set. The calculated results show that the state 1(2)Sigma(+) has the lowest CASPT2 energy among the electronic states. By means of the geometric optimization of this radical, it could be found that the molecule exhibits linear structure, with the bond lengths R-1 = 1.214 angstrom, R-2 = 1.363 angstrom, R-3 = 1.162 angstrom, which are very close to the experimental values. The calculated vertical excitation energies and the corresponding oscillator strengths show that there are three relatively strong peaks at energies 0.63, 4.04, and 5.49 eV, which correspond to the transitions 1(2)Sigma(+) -> 1(2)Pi, 1(2)Sigma(+)-> 2(2)Pi, and 1(2)Sigma(+) -> 2(2)Sigma(+), respectively. Additionally, the electronic configuration and the harmonic vibration frequencies of each state are also investigated.