The electronic spectrum of AgBr2: Ab initio benchmark calculations on the 2Πu and 2Σu+ charge transfer states including spin‐orbit effects

Abstract

AbstractThe 2Πu and 2Σu+ charge transfer (CT) states of AgBr2 have been studied through benchmark ab initio HF, complete active space SCF, CASPT2, NEVPT2, and averaged coupled pair functional (ACPF) multireference calculations using especially developed valence basis sets to study the transition energies, geometries, vibrational frequencies, and Mulliken charges. The spin‐orbit (SO) effects have been included through the effective Hamiltonian formalism using the ACPF energies as |ΛSΣ> diagonal elements. The equilibrium Ag–Br distance for both CT states is larger than that of the X2Πg ground state. The ACPF adiabatic excitation energies of the 2Πu and 2Σu+ states are 4620 and 7006 cm‐1, respectively. Using previous ACPF benchmark studies showing that the 2Δg ligand field (LF) state lies more than 20,000 cm‐1 above the ground state, these new results lead to the following energetic ordering for the five lowest electronic states of AgBr2: X2Πg, 2Σg+, 2Πu, 2Σu+, and 2Δg, in sharp contradiction with Hartree–Fock theory. When considering the coupling of the LF and CT states, the inclusion of the SO effects leads to a pure Ω = 3/2 (X2Πg) ground state, Ω = 1/2 (92% 2Πg + 8% 2Σg+) A state, Ω = 3/2 (pure 2Πu) B state, Ω = 1/2 (68% 2Πu+32% 2Σu+) C state, Ω = 1/2 (8% 2Πg + 92% 2Σg+) D state, and an Ω = 1/2 (32% 2Πu + 68% 2Σu+) E state. We present here what we consider the most complete and accurate account of the electronic spectrum of AgBr2. © 2012 Wiley Periodicals, Inc.