Calculation of Ligand Dissociation Energies in Large Transition-Metal Complexes

TL;DR

This study evaluates the accuracy of various computational methods, including coupled-cluster and density-functional approaches, for calculating ligand dissociation energies in large transition-metal complexes, highlighting the importance of dispersion corrections.

Contribution

It compares advanced ab initio and density-functional methods for ligand dissociation energies, identifying when single-reference coupled-cluster approaches are appropriate and assessing the impact of dispersion corrections.

Findings

Coupled-cluster methods agree with ab initio data for most reactions.

Density-functional methods improve with dispersion corrections.

Different dispersion correction schemes perform similarly well.

Abstract

Despite the importance of ligand dissociation energies for computational chemistry, obtaining accurate ab initio reference data is difficult and density-functional methods of uncertain reliability are chosen for feasibility reasons. Here, we consider advanced coupled-cluster and multi-configurational approaches to reinvestigate our WCCR10 set of ten gas-phase ligand dissociation energies. We assess the potential multi-configurational character of all molecules involved in these reactions in order to determine where single-reference coupled-cluster approaches can be applied. For some reactions of the WCCR10 set, large deviations from density-functional results including semiclassical dispersion corrections from experimental reference data had been observed. We tackle the issue (i) by comparing to ab initio data that comprise dispersion effects on a rigorous first-principles footing and (ii) by a comparison of density-functional approaches that model dispersion interactions in various ways. Only for two reactions, species exhibiting nonnegligible static electron correlation were identified, and hence, we may choose standard single-reference coupled-cluster approaches to compare with density-functional methods for the other eight reactions. For WCCR10, the Minnesota M06-L functional yielded the smallest mean absolute deviation without additional dispersion corrections in comparison to the coupled-cluster results and the PBE0-D3 functional produced the overall smallest mean absolute deviation. The agreement of density-functional results with coupled-cluster data increases significantly upon inclusion of any type of dispersion correction. It is important to emphasize that different density-functional schemes available for this purpose perform equally well. The coupled-cluster dissociation energies, however, deviate significantly from experimental results.