A simple approach to the state-specific MR-CC using the intermediate Hamiltonian formalism

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate prediction of molecular properties is crucial in chemistry.
• Multi-reference methods are needed for systems with strong electron correlation.
• Existing methods like CAS-CISD have limitations in accuracy.

Purpose of the Study:

• To develop a rigorous state-specific multi-reference coupled cluster (MRCC) formulation.
• To improve the accuracy of electronic structure calculations for challenging chemical systems.
• To provide a flexible and parallelizable computational tool.

Main Methods:

• Formulation of a state-specific MRCC method based on Meller et al.'s work.
• Utilizing Multi-Reference Singles and Doubles Configuration Interaction (MR-CISD) for initial amplitude guesses.
• Iterative diagonalization of a dressed MR-CISD matrix incorporating triples and quadruples.
• Testing on benchmark systems with Complete Active Spaces (CASs).

Main Results:

• The developed MRCC method achieves high accuracy, with errors of a few millihartree compared to full configuration interaction.
• Errors are approximately five times smaller than CAS-CISD.
• Deviation from strict separability is less than 10 μ hartree.

Conclusions:

• The new state-specific MRCC method offers a significant advancement in computational quantum chemistry.
• The method is accurate, efficient, parallelizable, and flexible for various applications.
• It provides a reliable tool for studying complex electronic structures and chemical reactions.