MBE-CASSCF Approach for the Accurate Treatment of Large Active Spaces.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Accurate electronic structure calculations are crucial for understanding molecular properties.
• Traditional methods struggle with large active spaces common in complex systems.

Purpose of the Study:

• Introduce a novel Many-Body Expansion-Full Configuration Interaction (MBE-FCI) based Complete Active Space Self-Consistent Field (CASSCF) method.
• Enable accurate electronic structure calculations for significantly larger active spaces.

Main Methods:

• Implemented a hybrid first-order algorithm for molecular orbital optimization.
• Combined Super-CI and quasi-Newton strategies.
• Utilized Many-Body Expansion-Full Configuration Interaction (MBE-FCI) for incremental approximation.

Main Results:

• Demonstrated computational efficacy and high accuracy of the MBE-CASSCF method.
• Validated the implementation through established numerical tests.
• Successfully applied MBE-CASSCF to study the spin gap of iron(II) porphyrin with large active spaces (50 electrons/50 orbitals).

Conclusions:

• The MBE-CASSCF method offers an efficient and accurate approach for electronic structure calculations in large systems.
• This method significantly expands the applicability of CASSCF to complex chemical problems.
• The study highlights the potential of MBE-CASSCF in investigating challenging molecular systems.