Using the GVB Ansatz to develop ensemble DFT method for describing multiple strongly correlated electron pairs.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Ensemble density functional theory (DFT) provides a framework for non-dynamic electron correlation in degenerate electronic states.
• Current spin-restricted ensemble-referenced KS (REKS) methods are limited to two fractionally occupied orbitals, restricting their application.
• Accurate description of polyradical species and multiple bond dissociations requires methods handling more than two fractionally occupied orbitals.

Purpose of the Study:

• To extend the REKS methodology to include multiple fractionally occupied Kohn-Sham (KS) orbitals.
• To develop a version of REKS capable of describing systems with four electrons in four fractionally occupied orbitals.
• To investigate the accuracy of the extended REKS method for strongly correlated molecules.

Main Methods:

• Integration of generalized valence bond (GVB) wavefunction theory with ensemble DFT.
• Derivation of a physically transparent energy expression dependent on fractional occupation numbers (FONs).
• Development and application of a four-electron, four-orbital REKS variant.

Main Results:

• A new REKS methodology enabling the description of systems with multiple fractionally occupied KS orbitals was derived.
• The accuracy of the extended REKS method was evaluated for various strongly correlated molecules.
• A scheme to address partial size-inconsistency arising from perfect spin-pairing was proposed.

Conclusions:

• The extended REKS method offers a pathway to more accurately describe complex electronic structures in chemistry.
• The proposed scheme may improve the reliability of ensemble DFT calculations for challenging molecular systems.
• Further research into perfect pairing natural orbital functionals in RDMFT is warranted to address size-consistency issues.