Coupled-cluster theory for the ground state and for excitations

TL;DR

This paper discusses the implementation and application of coupled-cluster methods for ground and excited states in molecular and periodic systems, highlighting recent advancements and computational tools.

Contribution

It introduces the CC-aims interface linking FHI-aims and Cc4s, enabling advanced correlated wave function methods for molecules and periodic systems.

Findings

Implementation of CC methods for periodic systems.

Application of EOM-CC for ionization and electron attachment.

Benchmark data for molecular and extended systems.

Abstract

In the molecular quantum chemistry community, coupled-cluster (CC) methods are well-recognized for their systematic convergence and reliability. The extension of the theory to extended systems has been comparably recent, so that developments and studies of periodic CC methods for both the ground-state and for excited states are still active fields of research and provide valuable benchmark data when the reliability of density functional approximations is questionable. In this contribution we describe the CC-aims interface between the FHI-aims and the Cc4s software packages. This linkage makes a variety of correlated wave function-based ground-state methods including M\o ller-Plesset perturbation theory (MP2), the random-phase approximation (RPA) and the gold-standard of quantum chemistry CCSD(T) method for both molecular and periodic applications accessible. This contribution discusses these ground-state methods for clusters and molecules, as well as for periodic systems. In particular, we discuss recent advancements and the implementation of the equation-of-motion CC method for the calculation of ionization (IP-EOM-CCSD) and electron attachment (EA-EOM-CCSD) processes. Open questions and routes to solutions are discussed as well.