Ab initio study on the spectroscopy of CuCl2. II. Benchmark calculations on the $X^{2}\Pi_{g} - C^{2}\Delta_{g}$ and $X^{2}\Pi_{g} - D^{2}\Delta_{g}$ transitions. - Archive ouverte HAL Access content directly
Journal Articles The Journal of Chemical Physics Year : 2004

Ab initio study on the spectroscopy of CuCl2. II. Benchmark calculations on the $X^{2}\Pi_{g} - C^{2}\Delta_{g}$ and $X^{2}\Pi_{g} - D^{2}\Delta_{g}$ transitions.

Abstract

The $X^{2}\Pi_{g} - C^{2}\Delta_{g}$ and $X^{2}\Pi_{g} - D^{2}\Delta_{g}$ transitions on CuCl2 have been studied using the most sophisticated non-dynamic and dynamic electronic correlation treatments. We report here ab initio benchmark calculations using especially developed basis sets to study, at the CASSCF+CASPT2 and CASSCF+ACPF levels, the transition energies as well as the corresponding equilibrium geometries. 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 $^{2}\Delta_{g} \Omega$=5/2 C 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 DFT-based descriptions for the $^{2}\Delta_{g}$ state, which largely overestimate its Te, 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.
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Dates and versions

hal-00003672 , version 1 (22-12-2004)

Identifiers

  • HAL Id : hal-00003672 , version 1

Cite

Alejandro Ramírez-Solís, Jean-Pierre Daudey. Ab initio study on the spectroscopy of CuCl2. II. Benchmark calculations on the $X^{2}\Pi_{g} - C^{2}\Delta_{g}$ and $X^{2}\Pi_{g} - D^{2}\Delta_{g}$ transitions.. The Journal of Chemical Physics, 2004, 122, pp.014315. ⟨hal-00003672⟩
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