Comment on "Direct and Converse Flexoelectricity in Two-Dimensional Materials"

In a recent letter [Phys. Rev. Lett. 127, 216801 (2021)], the authors introduced the flexovoltage coefficient $\varphi$ for quantifying the flexoelectric effect in 2D materials. In particular, they introduced the metric term $\varphi^\text{M}$ -- quadrupolar moment of the unperturbed charge density -- which they claim to have been neglected in the formulation of Ref. [Phys. Rev. Mat. 5, L030801 (2021)] for the flexoelectric coefficient $\mu_\text{T}$ (defined for short-circuit conditions), and used it to explain the significant disagreement in results. Here, we clarify that $\mu_\text{T}$ has been properly defined in Ref. [Phys. Rev. Mat. 5, L030801 (2021)], and that $\varphi \neq \mu_{\rm T}/\epsilon_0 + \varphi^\text{M}, \, \epsilon_0: \text{vacuum permittivity}$, contrary to the claims of Ref. [Phys. Rev. Lett. 127, 216801 (2021)].