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

D Földvári¹, Zs Tóth¹, P R Surján¹

¹Laboratory of Theoretical Chemistry, Faculty of Science, Institute of Chemistry, ELTE Eötvös Loránd University, Budapest, Hungary.

The Journal of Chemical Physics

|January 21, 2019

Summary

This study introduces a new perturbative correction method for multireference systems, improving accuracy by relaxing configuration interaction coefficients within unrestricted Hartree-Fock geminals. The approach shows promise for spin-pure wavefunctions but is sensitive to spin contamination.

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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.