The spin-polarized extended Brueckner orbitals

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Conventional natural and Brueckner orbitals (BOs) are standard for enhancing active orbital spaces in configuration interaction (CI) methods.
• These conventional models struggle to accurately represent highly correlated and quasidegenerate electronic states, such as open-shell singlets and dissociative states.

Purpose of the Study:

• To propose and define spin-polarized extended Brueckner orbitals (BOs) as an improvement over conventional BOs for describing challenging electronic states.
• To demonstrate the utility of extended BOs in analyzing highly correlated electronic states and overcoming limitations of existing single-determinant models.

Main Methods:

• Formal definition of spin-polarized extended Brueckner orbitals (BOs) based on Löwdin's spin-extended Hartree-Fock method.
• Development of a computational algorithm for calculating extended BOs within full CI and complete active space methodologies.
• Analysis of extended BOs using spin-up and spin-down density-like matrices to generate spin-polarization diagrams for unpaired electrons.

Main Results:

• Extended BOs are shown to always exist, unlike unrestricted BOs, offering greater flexibility for analyzing electronic states.
• Spin-projected determinant models using extended BOs demonstrate strong overlap with multi-configurational states, even for quasidegenerate systems and bond breaking.
• Calculations on hydrogen clusters and radicals confirm the effectiveness of extended BOs in capturing essential correlation effects.

Conclusions:

• Spin-polarized extended Brueckner orbitals provide a robust and flexible approach for accurately describing highly correlated and quasidegenerate electronic states.
• The proposed methodology enhances the capabilities of configuration interaction methods, particularly for systems exhibiting complex electronic behaviors like bond dissociation.