Ab initio calculations on the excited states of π-systems. I. Valence excitations in acetylene

The electronic ground state and the first valence excited states of acetylene (³Σ⁺_u, ^1,3Σ^-_u and ^1,3Δ_u) are investigated with the aid of ab initio methods applying large and flexible basis sets. Electron correlation has been taken into account on various levels of sophistication, including SCF CI, MC SCF and MC SCF CI methods. Equilibrium geometries are determined for all states mentioned, excitation energies, vertical and 0-0 transitions are reported and the dependence of the computed results on the type of wavefunction is discussed. Harmonic force constants and vibrational frequencies are calculated for all states treated in this paper and compared to experimental values.