Equation-of-Motion Coupled-Cluster Theory based on the 4-component Dirac-Coulomb(-Gaunt) Hamiltonian. Energies for single electron detachment, attachment and electronically excited states

TL;DR

This paper presents a relativistic 4-component EOM-CCSD implementation for calculating ionization potentials, electron affinities, and excitation energies, extending tensor contraction schemes and applying it to halogen oxides and related molecules.

Contribution

The work introduces an extended tensor contraction scheme for 4-component relativistic EOM-CCSD and applies it to complex molecules, comparing results with other methods.

Findings

Accurate ionization potentials and excitation energies for halogen oxides.

Extended tensor contraction scheme handles non-totally symmetric tensors.

Good agreement with multi-reference coupled-cluster results.

Abstract

We report in this paper an implementation of 4-component relativistic Hamiltonian based Equation-of-Motion Coupled-Cluster with singles and doubles (EOM-CCSD) theory for the calculation of ionization potential (IP), electron affinity (EA) and excitation energy (EE). In this work we utilize previously developed double group symmetry-based generalized tensor contraction scheme, and also extend it in order to carry out tensor contractions involving non-totally symmetric and odd-ranked tensors. Several approximated spin-free and two-component Hamiltonians can also be accessed in this implementation. We have applied this method to the halogen monoxide (XO, X= Cl, Br, I, At, Ts) species, in order to assess the quality of a few other recent EOMCC implementations, where spin-orbit coupling contribution has been approximated in different degree. Besides, we also have studied various excited states of CH$_2$IBr, CH$_2$I$_2$ and I$_2^-$(as well as single electron attachment and detachment electronic states of the same species) where comparison has been made with a closely related multi-reference coupled-cluster method, namely Intermediate Hamiltonian Fock Space Coupled-Cluster singles and doubles (IHFS-CCSD) theory.