The Delicate Balance of Static and Dynamic Electron Correlation

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Multi-configurational methods offer qualitative electronic structure descriptions but require multi-reference perturbation theory for quantitative accuracy.
• Accuracy is limited by active space size and perturbation theory specifics, while single-reference methods excel for equilibrium structures.

Purpose of the Study:

• To calculate the heterolytic double dissociation energy of four 3d-metallocenes.
• To compare complete active space self-consistent field (CASSCF) results with highly accurate coupled cluster (CC) data.
• To investigate the efficacy of information entropy measures for active space selection in CASSCF.

Main Methods:

• Complete Active Space Self-Consistent Field (CASSCF) method.
• Coupled Cluster (CC) calculations for benchmark data.
• Information entropy measures for active space selection.

Main Results:

• CASSCF calculations with entropy-based active space selection achieved accuracy comparable to coupled cluster methods.
• Coupled cluster data were well within experimental error bars.
• A subtle balance between static and dynamic electron correlation was observed, highlighting the need for algorithmic active space selection.

Conclusions:

• Entropy-based active space selection is a promising approach for accurate electronic structure calculations.
• Accurate calculation of dissociation energies in 3d-metallocenes requires careful consideration of electron correlation effects.
• Algorithmic active space selection methods are essential for reliable computational chemistry results.