Extending the Projection-Based Embedding Technique to Open-Shell Systems Using the Huzinaga Equation.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate electronic structure calculations are crucial for understanding chemical reactions.
• Multilevel methods are employed to balance accuracy and computational cost.
• Existing methods like ONIOM and MLC have limitations for open-shell systems.

Purpose of the Study:

• To present a novel projection-based embedding (PbE) approach for open-shell systems.
• To enable the combination of wave function theory (WFT) and density functional theory (DFT) methods.
• To develop a restricted formulation for PbE, extending previous work.

Main Methods:

• Implementation of the Huzinaga equation for both spin-restricted and spin-unrestricted calculations.
• Development of a new restricted PbE theory based on Roothaan and Shaik/Filatov's work.
• Testing of various PbE schemes with different subsystem sizes and high/low-level method combinations.

Main Results:

• The developed PbE method allows arbitrary combinations of restricted and unrestricted WFT/DFT methods.
• Error in reaction energies was analyzed based on the size of the high-level subsystem.
• Comparison with vacuum embedding, ONIOM, and MLC showed MLC as the best performer.
• Competitive PbE and ONIOM variants utilized generalized gradient approximation (GGA) low-level methods.

Conclusions:

• PbE offers a robust and systematically improvable alternative for multilevel calculations.
• The choice between PbE and ONIOM depends on specific accuracy and computational cost requirements.
• Further research is needed to fully elucidate the advantages of PbE over ONIOM.