The intrapair electron correlation in natural orbital functional theory

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The two-particle cumulant formulation is crucial for accurate electronic structure calculations.
• Previous methods like Piris natural orbital functional 5 (PNOF5) have limitations in handling complex electron correlation.
• Ensuring N-representability of the two-particle reduced density matrix (2-RDM) is essential for reliable theoretical predictions.

Purpose of the Study:

• To extend the Piris natural orbital functional (PNOF5) to include more than two natural orbitals.
• To develop an improved approximation for reconstructing the 2-RDM that satisfies positivity conditions (D, Q, G).
• To introduce the extended Piris natural orbital functional 5 (PNOF5e) for advanced quantum chemical applications.

Main Methods:

• Formulation of an extended two-particle cumulant based on an orbital-pairing scheme.
• Reconstruction of the 2-RDM using the new approximation, enforcing D, Q, and G positivity conditions.
• Application of PNOF5e to homolytic dissociation of H2, LiH, and Li2, and analysis of atomic clusters (Li2, Li3(+), Li4(2+), H3(+)) using Bader's theory.

Main Results:

• The derivation of the extended Piris natural orbital functional 5 (PNOF5e).
• Successful application of PNOF5e to model molecular dissociation and electron density in atomic clusters.
• Visualization of electron densities revealed improvements of PNOF5e over PNOF5.

Conclusions:

• PNOF5e offers a more accurate description of electron correlation compared to PNOF5.
• The extended functional provides a robust framework for N-representable 2-RDM calculations.
• The study demonstrates the potential of PNOF5e for accurate chemical predictions in molecular systems.