An orbital-invariant and strictly size extensive post-Hartree-Fock correlation functional.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Post-Hartree-Fock methods are crucial for accurate electronic structure calculations.
• Coupled-cluster singles doubles (CCSD) is a standard but computationally intensive method.
• Orbital invariance is a desirable property for correlation functionals.

Purpose of the Study:

• To present a novel, size-extensive post-Hartree-Fock correlation functional.
• To develop a functional invariant under occupied and virtual orbital transformations.
• To achieve CCSD-level accuracy with potentially reduced computational cost.

Main Methods:

• Development of a strictly size-extensive correlation functional.
• Ensuring invariance with respect to orbital transformations.
• Comparison of results with coupled-cluster singles doubles (CCSD) calculations.

Main Results:

• The presented functional reproduces CCSD results with high accuracy.
• The method avoids the need for additional Z-vector equations for properties and gradients.
• The functional can be systematically improved by including triple excitations, similar to CCSD.
• Orbital optimization increases computational cost compared to CCSD, but Bruecknerization reduces it below CCSD.

Conclusions:

• The new functional offers a rigorous and systematically improvable alternative to CCSD.
• It provides a balance between accuracy and computational efficiency, especially when using Brueckner conditions.
• This method advances the development of accurate and feasible post-Hartree-Fock correlation functionals.