Ab initio and DFT analysis of the low-lying electronic states of metal dihalides: quantum chemical calculations on the neutral BrMCl (M = Cu, Ag, Au).

The electronic configuration of the electronic ground and low-lying doublet excited states of neutral metal dihalides BrMCl (M = Cu, Ag, Au) has been investigated on the basis of CASSCF/CASPT2 methods taking into account scalar relativistic effects. A preliminary study of the electronic problem in BrAgCl, based on DFT and CASSCF/CASPT2 approaches and using various basis sets, namely relativistic all-electron basis sets, effective core potentials and ab initio model potentials (AIMP), as well as non-relativistic AIMP is discussed. It is shown that single-determinant methods are not flexible enough to describe the bonding of the neutral species in the electronic ground state regardless of the basis set. The failure to allocate the single electron of BrAgCl correlates with a wrong charge distribution within the complex, which is more accentuated when using pseudopotential basis sets. The inclusion of static and dynamic correlation effects by means of CASSCF/CASPT2 methods using large relativistic all-electron basis sets provides a correct qualitative picture of the electronic structure of the BrMCl series (M = Cu, Ag, Au). The spin unrestricted KS-DFT approach leads to a reasonable description of the degenerate electronic ground state ((2)Σ/(2)Π) bonding in these complexes with negligible spin contamination providing comparative spin densities in the series of molecules under investigation.