Coupled Cluster-Inspired Geminal Wavefunctions

TL;DR

This paper introduces new geminal wavefunctions inspired by coupled cluster theory, aiming to improve the accuracy and computational efficiency of describing strongly correlated electronic systems without complex orbital optimization.

Contribution

It develops a hierarchy of geminal wavefunctions based on the pCCD ansatz, incorporating single-like excitations and seniority restrictions, with benchmarking on model systems.

Findings

New geminal wavefunctions extend the pCCD framework.

Inclusion of single-like excitations enhances accuracy.

Benchmarks show promising results for strongly-correlated systems.

Abstract

Electron pairs have an illustrious history in chemistry, from powerful concepts to understanding structural stability and reactive changes, to the promise of serving as building blocks of quantitative descriptions of the electronic structure of complex molecules and materials. However, traditionally, two-electron wavefunctions (geminals), have not enjoyed the popularity and widespread use of the more standard single-particle methods. This has changed recently, with a renewed interest in the development of geminal wavefunctions as an alternative to describing strongly-correlated phenomena. Hence, there is a need to find geminal methods that are accurate, computationally-tractable, and do not demand significant input from the user (particularly, via cumbersome and often ill-behaved orbital optimization steps). Here we propose new families of geminal wavefunctions, inspired by the pair Coupled Cluster (pCCD) ansatz. We present a new hierarchy of two-electron wavefunctions that extend the one-reference orbital idea to other geminals. Moreover, we show how to incorporate single-like excitations in this framework, without leaving the quasiparticle picture. We explore the role of imposing seniority restrictions on these wavefunctions, and benchmark these new methods on model strongly-correlated systems.