# Origin of the Counterintuitive Dynamic Charge in the Transition-Metal Dichalcogenides

## TL;DR

This paper explores the unusual dynamical charge behavior in transition-metal dichalcogenides, revealing that their large and sometimes negative charges arise from local polarization effects and orbital hybridization, with implications for experimental verification.

## Contribution

It provides a theoretical explanation for the counterintuitive dynamical charges in transition-metal dichalcogenides based on polarization and orbital hybridization analysis.

## Key findings

- Dynamical charges are anomalously large in trigonal structures.
- In hexagonal structures, transition metals can have negative charges.
- A link between Born effective charge sign and π-backbonding is established.

## Abstract

We investigate the chemical bonding characteristics of the transition metal dichalcogenides based on their static and dynamical atomic charges within Density Functional Theory. The dynamical charges of the trigonal transition metal dichalcogenides are anomalously large, while in their hexagonal counterparts, their sign is even counterintuitive i.e. the transition metal takes the negative charge. This phenomenon cannot be understood simply in terms of a change in the static atomic charge as it results from a local change of polarization. We present our theoretical understanding of these phenomena based on the perturbative response of the system to a static electric field and by investigating the hybridization of the molecular orbitals near the Fermi level. Furthermore, we establish a link between the sign of the Born effective charge and the $\pi$-backbonding in organic chemistry and propose an experimental procedure to verify the calculated sign of the dynamical charge in the transition metal dichalcogenides.