Unitary coupled-cluster based self-consistent polarization propagator theory: a quadratic unitary coupled-cluster singles and doubles scheme

TL;DR

This paper introduces a quadratic unitary coupled-cluster singles and doubles (qUCCSD) method for self-consistent polarization propagator calculations, improving accuracy and robustness over previous schemes in electronic structure computations.

Contribution

The paper develops a novel quadratic UCCSD scheme with a simplified commutator truncation strategy for better ground and excited state predictions.

Findings

Significant accuracy improvement over UCC3 scheme.

Enhanced robustness in electronic property calculations.

Benchmark results show better excitation energy predictions.

Abstract

The development of a quadratic unitary coupled-cluster singles and doubles (qUCCSD) based self-consistent polarization propagator method is reported. We present a simple strategy for truncating the commutator expansion of the UCC transformed Hamiltonian $\bar{H}$. The qUCCSD method for the electronic ground-state includes up to double commutators for the amplitude equations and up to cubic commutators for the energy expression. The qUCCSD excited-state eigenvalue equations include up to double commutators for the singles-singles block of $\bar{H}$, single commutators for the singles-doubles and doubles-singles blocks, and the bare Hamiltonian for the doubles-doubles block. Benchmark qUCCSD calculations of the ground-state properties and excitation energies for representative molecules demonstrate significant improvement of the accuracy and robustness over the previous UCC3 scheme derived using M{\o}ller-Plesset perturbation theory.