Simple and efficient computational strategies for calculating orbital energies and pair-orbital energies from pCCD-based methods

TL;DR

This paper presents computationally affordable methods based on pCCD for calculating orbital and pair-orbital energies, enabling accurate predictions of ionization potentials, electron affinities, and related properties with low computational cost.

Contribution

It introduces a pCCD-based approach extending Koopmans' theorem for efficient and reliable orbital energy calculations in atomic and molecular systems.

Findings

pCCD natural orbitals provide balanced orbital energies

The methods accurately predict charge gaps and ionization potentials

Benchmark results show good agreement with experimental data

Abstract

We introduce affordable computational strategies for calculating orbital and pair-orbital energies in atomic and molecular systems. Our methods are based on the pair Coupled Cluster Doubles (pCCD) ansatz and its orbital-optimized variant. The computed orbital and pair-orbital energies are then subsequently used to approximate ionization potentials (IP), electron affinities (EA), the resulting charge gaps, double ionization potentials (DIP), and double electron affinities (DEA). Our methodology builds on the standard Koopmans' theorem and extends it for a pCCD-based wave function. Furthermore, we incorporate pCCD electron correlation effects into the model utilizing canonical Hartree-Fock or natural pCCD-optimized orbitals. The latter represents a diagonal approximation to the (D)IP/D(EA) equation of motion pCCD models. We benchmarked our newly developed models against theoretical and available experimental data for selected atoms in various basis set sizes and a set of 24 organic acceptor molecules. Our numerical results show that the Koopmans' approach based on pCCD natural orbitals provides a balanced treatment of occupied and virtual orbital energies, resulting in reliable predictions of charge gaps at a low computational cost.