# First-principles theory of spatial dispersion: Dynamical quadrupoles and flexoelectricity

## TL;DR

This paper develops a first-principles framework using density-functional perturbation theory to efficiently compute spatial dispersion effects, including flexoelectricity and dynamical quadrupoles, by treating wavevector as a perturbation parameter.

## Contribution

It introduces a novel long-wave expansion method that allows calculation of spatial dispersion effects from uniform perturbations, simplifying complex response property computations.

## Key findings

- Calculated flexoelectric tensors for crystalline insulators.
- Derived dynamical quadrupole tensors for model systems.
- Established a general framework for spatial dispersion analysis.

## Abstract

Density-functional perturbation theory (DFPT) is nowadays the method of choice for the accurate computation of linear and non-linear response properties of materials from first principles. A notable advantage of DFPT over alternative approaches is the possibility of treating incommensurate lattice distortions with an arbitrary wavevector, ${\bf q}$, at essentially the same computational cost as the lattice-periodic case. Here we show that ${\bf q}$ can be formally treated as a perturbation parameter, and used in conjunction with established results of perturbation theory (e.g. the "2n+1" theorem) to perform a long-wave expansion of an arbitrary response function in powers of the wavevector components. This provides a powerful, general framework to accessing a wide range of spatial dispersion effects that were formerly difficult to calculate by means of first-principles electronic-structure methods. In particular, the physical response to the spatial gradient of any external field can now be calculated at negligible cost, by using the response functions to $\mathit{uniform}$ perturbations (electric, magnetic or strain fields) as the only input. We demonstrate our method by calculating the flexoelectric and dynamical quadrupole tensors of selected crystalline insulators and model systems.