Ab initio study on the spectroscopy of CuCl2. II. Benchmark calculations on the X2Pi g-C2Deltag and X2Pi g-D2Deltag transitions.

The X (2)Pi(g)-C (2)Delta(g) and X (2)Pi(g)-D (2)Delta(g) transitions on CuCl(2) have been studied using the most sophisticated nondynamic and dynamic electronic correlation treatments. We report here ab initio benchmark calculations using especially developed basis sets to study, at the complete active space self-consistent field plus second-order Møller-Plesset algorithm (CASSCF+CASPT2) and CASSCF+ACPF levels, the transition energies as well as the corresponding equilibrium geometries (ACPF-averaged coupled pair functional). The spin-orbit (SO) effects of both atoms were included in a second step through the effective Hamiltonian formalism, using the calibrated SO effective potentials developed by the Stuttgart group. Without SO at the CASSCF+ACPF level, the vertical excitation energy for the (2)Delta(g) state is 6711 cm(-1) and the symmetric stretching equilibrium Cu-Cl distance is 4.04 a.u. The inclusion of the SO effects leads to a pure (2)Delta(g) Omega=5/2C state and a Omega=3/2 (0.7% (2)Pi(g),99.3% (2)Delta(g))D state. The calculated transition energies for the C and D states are 6340 and 8020 cm(-1), in good agreement with the spin-orbit splitting recent values from gas-phase and rare-gas matrix isolation laser induced fluorescence experiments. The present benchmark results show, as was recently done for the X (2)Pi(g)-(2)Sigma(g) transition, the rather poor performance of all the density functional theory-based descriptions for the (2)Delta(g) state, which largely overestimate its T(e), systematically placing it around 19 000 cm(-1). The CASSCF+CASPT2 method also overestimates, by around 50%, the X (2)Pi(g)-(2)Delta(g) transition energy, showing that only large variational calculations can produce reliable spectroscopic results for this kind of complex systems where delicate electronic correlation effects have to be carefully dealt with.