Geminal perturbation theory based on the unrestricted Hartree-Fock wavefunction.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Describing multireference systems is challenging in quantum chemistry.
• Unrestricted Hartree-Fock (UHF) captures static correlation but requires further refinement.
• Perturbative corrections are essential for accurate electronic structure calculations.

Purpose of the Study:

• To develop a novel perturbative correction method for multireference systems.
• To leverage natural orbitals and pair function structure within the UHF framework.
• To improve the description of static correlation in electronic structure calculations.

Main Methods:

• A perturbative correction exploiting natural orbitals and UHF pair function structure.
• Relaxation of configuration interaction coefficients of UHF geminals at zero order.
• Utilizing a Dyall-type zero-order Hamiltonian with explicit two-electron interactions.
• Reducing computational scaling by adopting an effective one-electron zero-order Hamiltonian for virtual orbitals.

Main Results:

• The proposed method offers a simple framework for multireference systems.
• Formal properties of the perturbative schemes were analyzed.
• Energetic data and natural occupation numbers were used for assessment.
• Good performance was observed for systems with essentially spin-pure reference wavefunctions.

Conclusions:

• The perturbative correction method shows potential for describing multireference systems.
• Spin contamination in the UHF wavefunction at zero order can negatively impact the correction's performance.
• Further investigation into handling spin contamination is warranted for broader applicability.