Electronic spectra of the linear magnesium-containing carbon chains MgC2nH (n = 1-5): A CASPT2 study

Complete active space self-consistent-field (CASSCF) approach has been used for the geometry optimization of the X2Σ+ and A2Π electronic states for the linear magnesium-containing carbon chains MgC2nH (n = 1-5). Multireference second-order perturbation theory (CASPT2) has been used to calculate the vertical excitation energies from the ground to selected seven excited states, as well as the potential energy curves of two 2Σ+ and two 2Π electronic states. The studies indicate that the vertical excitation energies of the A2Π ← X2Σ+ transition for MgC2nH (n = 1-5) are 2.837, 2.793, 2.767, 2.714, and 2.669 eV, respectively, showing remarkable linear size dependence. Compared with the previous TD-DFT and RCCSD(T) results, our estimates for MgC2nH (n = 1-3) are in the best agreement with the available observed data of 2.83, 2.78, and 2.74 eV, respectively. In addition, the dissociation energies in MgC2nH (n = 1-5) are also been evaluated.