The Spectroscopy of Copper and Silver Monohalides: What Modern Quantum Chemistry Can and Cannot do

Abstract

A comprehensive analysis on the successes and failures of the theoretical spectroscopy of copper and silver monohalides is presented in light of the recent theoretical versus experimental information for these systems. It is shown that although for copper monohalides the standard quantum chemical multireference SCF, perturbational or CI approaches work well to describe their spectroscopy, for their silver counterparts this is not the case always. While density functional theory is intrinsically unable to deal with the most intense transition (B1Σ+–X1Σ+) observed in the whole CuX and AgX series, the CASSCF+CASPT2 method fails for the AgX family is describing this excitation, even with the largest physically meaningful valence orbital active space. The complexity of silver halides arises from the concatenation of three independent problems: first, the isolated atom spectroscopy is much more complex for Ag than for Cu, since for silver the Rydberg 2P(4d105p1) state lies close but below the valence 2D(4d95s2) one; secondly, the spin-orbit coupling for silver is much larger and thus the fine-structure components of these doublets are intertwined, leading to very large ΛSΣ mixing effects in the molecular case. Finally, rather strong interactions exist between the numerous Ag(4d95s2)X(np5) and Ag(4d105p1)X(np5) neutral configurations with the Ag+(4d95s1)X−(np6) ionic structures, making the accurate description of these mixtures an extremely difficult task, especially for the second 1Σ+ state, where even large CASSCF calculations fail at providing continuous state-specific potential energy curves. This remains a true theoretical impasse related to the still unsolved and complex convergence for the coupled CI-orbital multivariate minimization problem. Therefore, it is only possible to properly describe the electronic structure of the excited states of silver monohalides using the full valence active space, to perform variational treatments of the non-dynamic and dynamic electronic correlation treatments with especially optimized and extended RECP-basis sets; then the spin-orbit effects must imperatively be taken into account to qualitatively explain the nature of the observed AgX spectra. However, non-negligible errors are found for some of the basic spectroscopic quantities such as transition energies and vibrational frequencies for the most spectroscopically active excited states.