The performance of approximate equation of motion coupled cluster for valence and core states of heavy element systems

TL;DR

This study evaluates the accuracy of approximate equation of motion coupled cluster methods for heavy element systems, comparing their performance to the more accurate EOM-CCSD, especially for valence and core states relevant in atmospheric and astrophysical contexts.

Contribution

The paper implements and assesses approximate EOM-CC methods for four-component Hamiltonians, highlighting their accuracy relative to EOM-CCSD for heavy element systems.

Findings

Partitioned EOM-CCSD shows largest deviations from EOM-CCSD.

Second-order methods generally agree better with EOM-CCSD.

Imbalance due to lack of excited state relaxation affects accuracy.

Abstract

The equation of motion coupled cluster singles and doubles model (EOM-CCSD) is an accurate, black-box correlated electronic structure approach to investigate electronically excited states and electron attachment or detachment processes. It has also served as a basis for developing less computationally expensive approximate models such as partitioned EOM-CCSD (P-EOM-CCSD), the second-order many-body perturbation theory EOM (EOM-MBPT(2)), and their combination (P-EOM-MBPT(2)) [S. Gwaltney et al., Chem. Phys. Lett. 248, 189-198 (1996)]. In this work we outline an implementation of these approximations for four-component based Hamiltonians and investigate their accuracy relative to EOM-CCSD for valence excitations, valence and core ionizations and electron attachment, and this for a number of systems of atmospheric or astrophysical interest containing elements across the periodic table. We have found that across the different systems and electronic states of different nature considered, partition EOM-CCSD yields results with the largest deviations from the reference, whereas second-order based approaches tend show a generally better agreement with EOM-CCSD. We trace this behavior to the imbalance brought about by the removal of excited state relaxation in the partition approaches, with respect to degree of electron correlation recovered.