# Modeling the electronic structures of the ground and excited states of the ytterbium atom and the ytterbium dimer: A modern quantum chemistry perspective

## TL;DR

This paper provides a comprehensive quantum chemistry analysis of the electronic structures of ytterbium atom and dimer, including ground and excited states, using advanced methods to benchmark spectroscopic parameters and explore complex electronic spectra.

## Contribution

It introduces a detailed computational study employing multiple modern quantum chemistry methods, including pCCD-based approaches, to accurately model Yb and Yb$_2$ electronic states and spectra.

## Key findings

- Benchmark spectroscopic parameters for Yb$_2$
- Complex electronic spectrum of Yb$_2$ unraveled
- Bi-excited character of the first excited state analyzed

## Abstract

We present a comprehensive theoretical study of the electronic structures of the Yb atom and the Yb$_2$ molecule, respectively, focusing on their ground and lowest-lying electronically excited states. Our study includes various state-of-the-art quantum chemistry methods such as CCSD, CCSD(T), CASPT2 (including spin--orbit coupling), and EOM-CCSD as well as some recently developed pCCD-based approaches and their extensions to target excited states. Specifically, we scan the lowest-lying potential energy surfaces of the \ce{Yb2} dimer and provide a reliable benchmark set of spectroscopic parameters including optimal bond lengths, vibrational frequencies, potential energy depths, and adiabatic excitation energies. Our in-depth analysis unravels the complex nature of the electronic spectrum of \ce{Yb2}, which is difficult to model accurately by any conventional quantum chemistry method. Finally, we scrutinize the bi-excited character of the first $^1\Sigma_g^+$ excited state and its evolution along the potential energy surface.