1D Transition Metal Oxide Chains as a Challenging Model for Ab Initio Calculations

TL;DR

This study investigates one-dimensional transition metal mono-oxide chains using advanced computational methods, highlighting the challenges in finding global minima and comparing magnetic and electronic properties across different theoretical approaches.

Contribution

It provides a comprehensive comparison of DFT, DFT+U, and CCSD methods for modeling 1D transition metal oxides, revealing limitations and insights into magnetic states and band gap predictions.

Findings

DFT+U successfully predicts insulating states with band gaps.

Multiple local minima complicate ab initio calculations for these chains.

CCSD predicts larger energy differences and different magnetic ground states than DFT+U.

Abstract

Providing highly simplified models of strongly correlated electronic systems that challenge {\it ab initio} calculations can serve as a valuable testing ground to improve these methods. In this study, we present a comprehensive study of the structural, magnetic, and electronic properties of one-dimensional transition metal mono-oxide chains (VO, CrO, MnO, FeO, CoO, and NiO) using density functional theory (DFT), DFT+$U$, and coupled-cluster singles and doubles (CCSD) calculations. The Hubbard $U$ parameter for DFT+$U$ is determined using the linear response theory. In all systems studied except MnO, the presence of multiple local minima -- primarily due to the electronic degrees of freedom associated with the d-orbitals -- leads to significant challenges for DFT, DFT+U, and Hartree-Fock methods in finding the global minimum in ab initio calculations. Our results indicate that the antiferromagnetic (AFM) state is energetically favored for all chains, except CrO, when using DFT+$U$ and PBE. We analyze the electronic band structures and find that while the PBE approximation often predicts metallic or half-metallic ground states for the ferromagnetic (FM) state, DFT+$U$ approach successfully opens band gaps, correctly predicting insulating behavior in all cases. Furthermore, we compared the energy differences between the AFM and FM states using DFT+$U$ and CCSD for CrO, MnO, FeO, CoO and NiO. Our findings indicate that CCSD predicts larger energy differences in some cases compared to DFT+$U$, suggesting that the Hubbard $U$ parameter obtained through linear response theory may be overestimated when used to calculate energy differences between different magnetic states. For CrO, CCSD predicts an AFM ground state, in contrast to the predictions from DFT+$U$ and PBE methods.