Downfolded Configuration Interaction for Chemically Accurate Electron Correlation.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate calculation of electron correlations is crucial for understanding molecular properties.
• Existing methods often struggle with computational cost for large, realistic molecules.
• Developing efficient and accurate quantum chemical methods remains a key challenge.

Purpose of the Study:

• To develop a novel model subspace configuration interaction (CI) method.
• To achieve chemically accurate electron correlations for ground and excited states.
• To demonstrate the method's efficiency and applicability to complex molecular systems.

Main Methods:

• Constructing a compact effective Hamiltonian through iterative downfolding into a model subspace.
• Utilizing local clusters of Slater determinants and low-rank matrix properties.
• Avoiding direct selection and ordering of configurations in the full Hilbert space.

Main Results:

• The method achieves near-Full CI (FCI) accuracy for ground and excited-state potential energies.
• Benchmarking on the C2 molecule demonstrates the method's efficiency and accuracy.
• Successful application to organometallic complexes and diverse organic molecules is illustrated.

Conclusions:

• The developed model subspace CI method offers a computationally efficient route to accurate electron correlation.
• This approach is promising for calculating excitation energies in complex chemical systems.
• The method provides a robust tool for advanced quantum chemical investigations.