A new internally contracted multi-reference configuration interaction method.

Area of Science:

• Computational chemistry
• Quantum chemistry
• Electronic structure theory

Background:

• Accurate electronic structure calculations are crucial for understanding molecular properties.
• Existing multi-reference configuration interaction (MRCI) methods face challenges with large active spaces and many closed-shell orbitals.

Purpose of the Study:

• To develop a new, efficient internally contracted multi-reference configuration interaction (MRCI) method.
• To enable high-accuracy MRCI calculations for larger and more complex molecular systems.

Main Methods:

• Developed a new internally contracted MRCI method.
• Explicitly treated closed-shell orbitals, making dependencies solely on active orbital labels.
• Utilized a domain-specific computer algebra system for equation derivation.
• Employed an integrated tensor framework for semi-automatic implementation.

Main Results:

• The new MRCI method efficiently handles large active spaces, long configuration expansions, and numerous closed-shell orbitals.
• Achieved computational efficiency comparable to closed-shell single-reference methods for closed-shell orbitals.
• Demonstrated accuracy and efficiency with applications to dioxygen-copper complexes and ferrocene.

Conclusions:

• The developed MRCI method significantly expands the scope of high-accuracy quantum chemical calculations.
• This advancement facilitates the study of larger molecules and complex chemical phenomena.