Size-extensive seniority-zero energy functionals derived from configuration interaction with double excitations.

Area of Science:

• Quantum chemistry
• Computational physics
• Materials science

Background:

• The doubly occupied configuration interaction (DOCI) approach accurately describes nondynamic electron correlation but is computationally expensive.
• Pair coupled-cluster doubles (pCCD) offers a more efficient alternative, but its accuracy can be limited.
• Seniority-zero methods provide approximations to DOCI at lower computational costs.

Purpose of the Study:

• To develop and evaluate seniority-zero formulations of quantum chemistry methods.
• To compare the accuracy and efficiency of these new methods against established approaches like pCCD and DOCI.
• To investigate the suitability of the pair two-electron reduced density matrix (pp2RDM) approach for approximating DOCI energies.

Main Methods:

• Development of seniority-zero formulations for coupled-electron pair approximation, averaged coupled-pair functional, averaged quadratic coupled-cluster, and p2RDM.
• Implementation and testing of these methods on molecular systems.
• Comparison of calculated energies and reduced density matrices (RDMs) with DOCI and pCCD results.

Main Results:

• The pp2RDM approach yields energies comparable in quality to pCCD and DOCI.
• Seniority-zero Hermitian methods offer advantages over pCCD, including fewer parameters and simpler RDMs.
• pp2RDM-derived RDMs provide better approximations to DOCI RDMs than those from pCCD.

Conclusions:

• The pp2RDM method is a promising, computationally efficient approach for accurately describing nondynamic electron correlation.
• This work advances the development of accurate and scalable quantum chemistry methods.
• pp2RDM offers a superior alternative to pCCD for approximating DOCI results in molecular systems.