Abstract
We present a comprehensive theoretical study of the physical phenomena that determine the relative energies of three of the lowest electronic states of each of the square-planar copper complexes [CuCl4]2-, [Cu(NH3)4]2+, and [Cu(H2O)4]2+ and present a detailed analysis of the extent to which truncated configuration interaction (CI) and coupled cluster (CC) theories succeed in predicing the excitation energies. We find that ligand-metal charge transfer (CT) single excitations play a crucial role in the correct determination of the properties of these systems, even though the first impact of these CT on the energetics of these systems appears at fourth-order in perturbation theory. We provide a minimal selected CI space for describing these systems with multireference theories and use a high-order perturbation theory analysis within this space to derive a simple and general physical picture for the LMCT process. We find that coupled cluster singles and doubles (CCSD) energy differences agree very well with near full CI values even though the D1 diagnostics are large, which casts doubt on the usefulness of single-amplitude-based multireference diagnostics. Configuration interaction singles and doubles (CISD) severely underestimates the excitation energies, and the failure is a direct consequence of the size-inconsisency errors in CISD. Finally, we present reference values for the energy differences computed using explicitly correlated CCSD(T) and BCCD(T) theory.
Highlights
Open-shell transition metal complexes, which are ubiquitous in biological and industrial chemistry, represent one of the main challenges for present-day quantum chemistry, where theory seeks to provide prediction and interpretation of key properties such as electronic transition energies, spin-density maps and magnetic anisotropy
Through careful benchmarking and theoretical analysis, this work highlights that the correct theoretical determination of the electronic spectroscopy and the ground state spin density of open-shell transition metal complexes requires methods that correctly couple a range of correlation processes
Definitive reference energies and wave functions for the three low-lying spin states of [CuCl4]2−, [Cu(NH3)4]2+, and [Cu(H2O)4]2+, in a modest 6-31G basis, were obtained from near-full configuration interaction (FCI) calculations performed using the CIPSI selected configuration interaction (CI) method. Analysis of these states revealed the prevalence of a specific excited configuration in all of the computed wave functions, which plays a decisive role in the spin density and energies of the spin states
Summary
Open-shell transition metal complexes, which are ubiquitous in biological and industrial chemistry, represent one of the main challenges for present-day quantum chemistry, where theory seeks to provide prediction and interpretation of key properties such as electronic transition energies, spin-density maps and magnetic anisotropy. In multireference computational studies of two of these systems,[9,14] Neese et al and Pierloot et al observed that in order to correctly describe the electronic spectrum and magnetic properties, it is necessary to include the ligand donor orbital in the active space, even though this orbital is doubly occupied and has a relatively low orbital eigenvalue. They found that CASPT2 performs poorly and sophisticated methods such as SORCI9 or MS-CASPT214 are required, which couple the dynamic correlation into the multireference treatment. A common observation in all of these studies is that
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