# The Electronic Structure of Planar Rhombic Co2O2

## TL;DR

This paper investigates the electronic structure of planar rhombic Co2O2 using various computational methods to determine its ground state and magnetic properties.

## Contribution

The study reveals conflicting predictions of Co2O2's ground state from different computational methods and identifies the importance of correlation effects.

## Key findings

- DFT, CCSD(T), and CASSCF calculations suggest a high-spin septet ground state for Co2O2.
- CASPT2 calculations indicate a singlet ground state (1A_g) with significant static and dynamic correlation effects.
- MICD susceptibility calculations show a strong diatropic ring current in the 1A_g state with contributions from σ and π orbitals.

## Abstract

Low-lying valence states of the planar rhombic (D
2h) structure of Co2O2 have been
computationally studied at density functional theory (DFT) levels,
at the singles and doubles coupled-cluster level augmented with a
perturbative treatment of the triples (CCSD­(T)), at the complete active
space self-consistent field (CASSCF) level, and at the second-order
complete active space perturbation theory (CASPT2) level. Calculations
at the DFT, CCSD­(T), and CASSCF levels suggest that the ground state
is a high-spin septet state, whereas the CASPT2 calculations yield
a singlet ground state (1
A

g
). The wave function of the CCSD­(T) ground state
(7
B
2u
) is
dominated by one Slater determinant and is therefore well described
using single-reference methods, whereas the configuration interaction
coefficient (CI) of the reference determinant (C
0) of the 1
A

g
state is only 0.519 at the CASSCF level. High-spin states
are stabilized at the DFT level by increasing the amount of Hartree–Fock
(HF) exchange in the functional. Static and dynamic correlation effects
must be considered to obtain the correct ground state of Co2O2. DFT calculations of the magnetically induced current
density (MICD) susceptibility of the 1
A

g
state using the TPSSh functional show
that the molecule sustains a strong diatropic ring current, which
has significant contributions from both σ and π orbitals.