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The X 2Πg, 2Σg+, and 2Δg states of AgCl2 have been studied through benchmark ab initio complete active space self-consistent field plus second-order complete active space multireference Möller-Plesset algorithm (CASSCF+CASPT2) and complete active space self-consistent field plus averaged coupled pair functional (CASSCF+ACPF) and density-functional theory (DFT) calculations using especially developed basis sets to study the transition energies, geometries, vibrational frequencies, Mulliken charges, and spin densities. The spin-orbit (SO) effects were included through the effective Hamiltonian formalism using the ΛSΣ ACPF energies as diagonal elements. At the ACPF level, the ground state is 2Πg in contradiction with ligand-field theory, SCF, and large CASSCF; the adiabatic excitation energies for the 2Σg+ and 2Δg states are 1640 and 18 230 cm-1, respectively. The inclusion of the SO effects leads to a pure Ω=3/2(2Πg) ground state, a Ω=1/2 (66%2Πg and 34%2Σg+) A state, a Ω=1/2 (34%2Πg and 66%2Σg+) B state, a Ω=5/2(2Δg)C state, and a Ω=3/2(99%2Δg)D state. The X-A, X-B, X-C, and X-D transition energies are 485, 3715, 17 246, and 20 110 cm-1, respectively. The B97-2, B3LYP, and PBE0 functionals overestimate by ≈100% the X 2Πg-2Σg+Te but provide a qualitative energetic ordering in good agreement with ACPF results. B3LYP with variable exchange leads to a 42% optimal Hartree-Fock exchange for transition energies but all equilibrium geometries get worsened. Asymptotic corrections to B3LYP do not provide improved values. The nature of the bonding in the X 2Πg state is very different from that of CuCl2 since the Mulliken charge on the metal is 1.1 while the spin density is only 0.35. DFT strongly delocalizes the spin density providing even smaller values of around 0.18 on Ag not only for the ground state, but also for the 2Σg+ state. [ABSTRACT FROM AUTHOR]

The X²Πg-²Σg,X²Πg-²Δg, X²Πg-²Σu, X²Πg-²Πu transitions on CuCI2 have been studied using several exchange-correlation functionals from the various types of density functional theory (DET) approaches like local density approximation (LDA), generalized gradient approximation (GGA), hybrid and meta-GGA. The results are compared with the experience and with those coming from the most sophisticated nondynamic and dynamic electronic correlation treatments using the same relativistic effective core potentials and especially developed basis sets to study the electronic structure of the five lowest states and the corresponding vertical and adiabatic transition energies. The calculated transition energies for three of the hybrid functionals (B3LYP, B97-2, and PBE0) are in very good agreement with the benchmark ab initio results and experimental figures. All of the other functionals largely overestimate the X²Πg-²Σg and X²Πg-²Δgtransition energies, many of them even placing the ²Δg ligand field state above the charge transfer ²Πu and ²Σu+ states. The relative weight of the Hartree-Fock exchange in the definition of the functional used appears to play a key role in the accurate description of the ASΣ density defined by the orientation of the 3d hole (σ,π,or δ) on Cu in the field of both chlorine atoms, hut no simple connection of this weight with the quality of the spectra has been found. Mulliken charges and spin densities are carefully analyzed; a possible link between the extent of spin density on the metal for the X²Πg state and the performance of the various functionals was observed, suggesting that those that lead to the largest values (close to 0.65) are the ones that best reproduce these four transitions. Most functionals lead to a remarkably low ionicity for the three ligand field states even for the best performing functionals, compared to the complete active space (SCF) (21, 14) ab initio values. These findings show that not only large variational ab initio calculations Can produce reliable spectroscopic results for extremely complex systems where delicate electronic correlation effects have to he carefully dealt with. However, those functionals that were recently shown to perform best for a series of molecular properties. [ABSTRACT FROM AUTHOR]