Tailored coupled cluster theory in varying correlation regimes.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Coupled cluster (CC) theory is a standard for accurate electronic structure.
• Multi-reference methods are needed for complex systems but are computationally expensive.
• Tailored coupled cluster (TCC) aims to combine the strengths of both approaches.

Purpose of the Study:

• To analyze the tailored coupled cluster (TCC) wave function.
• To assess TCC's reliability against exact wave functions for systems with static and dynamic correlation.
• To investigate factors affecting TCC accuracy, including active space size and reference wave function choice.

Main Methods:

• Detailed analysis of TCC wave function using model systems.
• Comparison of TCC results with exact (full configuration interaction) wave functions.
• Evaluation of electronic energy errors and wave function overlap as quality measures.
• Investigation of size-consistency and symmetry breaking effects.

Main Results:

• TCC shows promise for describing static and dynamic correlation.
• Errors from symmetry breaking can be reduced by choosing an appropriate reference determinant.
• Large active spaces, including orbitals with high single-orbital entropy, improve TCC accuracy.
• Larger active spaces can mitigate size-consistency errors.

Conclusions:

• TCC is a viable and promising approach for accurate electronic structure calculations.
• Careful selection of the active space and reference function is crucial for optimal TCC performance.
• TCC, especially with modern active-space techniques, can handle complex electronic correlations effectively.