Multireference Character for 4d Transition Metal-Containing Molecules

Four diagnostic criteria have been examined to identify the suitability of single-reference wave function-based quantum chemistry methods for a set of 118 4d transition metal species. These diagnostics include the weight of the leading configuration of the CASSCF wave function, C-0(2); the Frobenius norm of the coupled cluster amplitude vector related to single excitations, T-1; the matrix 2-norm of the coupled cluster T-1 amplitude vector arising from coupled cluster calculations, D-1; and the percent total atomization energy, %TAE, corresponding to a relationship between energies determined with CCSD and CCSD(T) calculations. New criteria, namely, T-1 >= 0.045, D-1 >= 0.120, and %TAE >= 10%, are herein proposed as a gauge for 4d transition metal-containing molecules to predict the possible need to employ multireference (MR) wave function-based methods to describe energetic and spectroscopic properties.