A Fock-operator complete active space self-consistent field (CAS-SCF) method combined with frozen-density embedding

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate treatment of electronic states in complex molecular systems is computationally demanding.
• Existing methods may struggle with multi-reference character and spin polarization.
• Efficient embedding methods are crucial for studying large molecules.

Purpose of the Study:

• To implement a Fock-operator complete-active space self-consistent field (CAS-SCF) method combined with frozen-density embedding (FDE) into the KOALA quantum-chemistry program.
• To extend the FDE contribution to the unrestricted case to account for spin polarization.
• To enable efficient calculation of multiple electronic states in complex molecular environments.

Main Methods:

• Configuration interaction from an unrestricted reference determinant.
• State-average (SA) CAS-SCF calculations.
• Frozen-density embedding (FDE) for molecular environments.
• Assessment using a stretched lithium dimer and retinal molecule.

Main Results:

• Successful implementation of CAS-SCF with FDE in the KOALA program.
• Demonstrated ability to treat singlet, triplet, and quintet states.
• Extended FDE to the unrestricted case, accounting for spin polarization.
• Efficient orbital optimization for 25 states in a retinal molecule calculation.
• FDE ansatz facilitates orbital localization and selection for CAS methods.

Conclusions:

• The implemented CAS-SCF/FDE method is efficient and accurate for studying electronic states in complex systems.
• The method effectively handles multi-reference character and spin polarization.
• This approach aids in the analysis of complex molecular environments by localizing orbitals on the target system.