The Delicate Balance of Static and Dynamic Electron Correlation

TL;DR

This paper investigates the balance between static and dynamic electron correlation in quantum chemistry, demonstrating that entropy-based active space selection improves accuracy in multireference calculations for transition metal complexes.

Contribution

It introduces an entropy-based method for active space selection that enhances the accuracy of multireference calculations compared to traditional restricted active space approaches.

Findings

Coupled cluster results agree with experimental data within error bars.

Entropy-based active space selection approaches the accuracy of coupled cluster methods.

A subtle balance exists between static and dynamic correlation effects, influencing active space choice.

Abstract

Multi-configurational approaches yield universal wave function parameterizations that can qualitatively well describe electronic structures along reaction pathways. For quantitative results, multi-reference perturbation theory is required to capture dynamic electron correlation from the otherwise neglected virtual orbitals. Still, the overall accuracy suffers from the finite size and choice of the active orbital space and peculiarities of the perturbation theory. Fortunately, the electronic wave functions at equilibrium structures of reactants and products can often be well described by single-reference methods and hence are accessible to accurate coupled cluster calculations. Here, we calculate the heterolytic double dissociation energy of four 3d-metallocenes with the complete active space self-consistent field method and compare to highly accurate coupled cluster data. Our coupled cluster data are well within the experimental error bars. This accuracy can also be approached by complete active space calculations with an orbital selection based on information entropy measures. The entropy based active space selection is discussed in detail. We find a very subtle balance between static and dynamic electron correlation effects that emphasizes the need for algorithmic active space selection and that differs significantly from restricted active space results for identical active spaces reported in the literature.