The electronic spectrum of AgCl2: Ab initio benchmark versus density-functional theory calculations on the lowest ligand-field states including spin-orbit effects

摘要

The X (2)Pi(g), (2)Sigma(g)(+), and (2)Delta(g) states of AgCl2 have been studied through benchmark ab initio complete active space self-consistent field plus second-order complete active space multireference Moller-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 Lambda S Sigma ACPF energies as diagonal elements. At the ACPF level, the ground state is (2)Pi(g) in contradiction with ligand-field theory, SCF, and large CASSCF; the adiabatic excitation energies for the (2)Sigma(g)(+) and (2)Delta(g) states are 1640 and 18 230 cm(-1), respectively. The inclusion of the SO effects leads to a pure Omega=3/2((2)Pi(g)) ground state, a Omega=1/2 (66%(2)Pi(g) and 34%(2)Sigma(g)(+)) A state, a Omega=1/2 (34%(2)Pi(g) and 66%(2)Sigma(g)(+)) B state, a Omega=5/2((2)Delta(g))C state, and a Omega=3/2(99%(2)Delta(g))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 approximate to 100% the X (2)Pi(g)-(2)Sigma(g)(+)T(e) 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 (2)Pi(g) 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 (2)Sigma(g)(+) state.