Interacting pairs in natural orbital functional theory

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Natural Orbital Functionals (NOFs) are essential for approximating the electron density and predicting molecular properties.
• Existing NOFs like PNOF5 (independent-pair approach) have limitations in capturing complex electron correlation effects.
• The PNOFi family of NOFs aims to improve accuracy by incorporating interacting-pair models.

Purpose of the Study:

• To develop and introduce a new interacting-pair natural orbital functional, termed PNOF6.
• To enhance the treatment of both dynamic and static electron correlations in quantum chemical calculations.
• To assess the performance of PNOF6 against its predecessor, PNOF5, using molecular dissociation and deformation as benchmarks.

Main Methods:

• Development of PNOF6 based on an interacting-pair model within the JKL-only NOF framework.
• Reconstruction of the two-particle reduced density matrix (2-RDM) from the one-particle RDM using explicit two-particle cumulants.
• Enforcement of (2,2)-positivity conditions for N-representability of the 2-RDM.
• Application to the dissociation of diatomic molecules and the deformation of benzene.

Main Results:

• PNOF6 demonstrates superior performance compared to PNOF5, effectively treating both dynamic and static correlations.
• Calculations for diatomic molecules yielded equilibrium distances, dipole moments, harmonic frequencies, anharmonicity constants, and binding energies.
• PNOF6 correctly predicted the D6h symmetry for benzene, unlike PNOF5 which introduced a symmetry-breaking distortion.

Conclusions:

• PNOF6 represents a significant advancement in NOF theory, combining the strengths of previous models.
• The new functional accurately describes molecular properties and electron correlation effects.
• PNOF6's ability to preserve molecular symmetry, as shown in the benzene deformation, highlights its improved accuracy and reliability.