Category:Dielectric properties

The dielectric function ${\displaystyle \epsilon(\mathbf q,\omega)}$ descrives the electric response of a system when perturbed by an external electric field.

Depending on the origin of the perturbing electric field, ${\displaystyle \epsilon(\mathbf q,\omega)}$ can be computed in different ways. At the present, VASP is able to compute the dielectric function from phonons using Density Functional Perturbation Theory (DFPT), or from the optical response at different levels of interaction between electrons and holes (e.g. Random Phase approximation and Bethe-Salpeter equations).

Dielectric response from atomic oscillations (DFPT)

Dielectric response from optics (non interacting case, RPA, and BSE)

Here the system is assumed to be under a weak electric field, which can be often referred to as a probe field in experiments. The external field is weak enough that it does not create enough excitations so that the system is driven into an excited state. This means that quantities such as the optical absorption can be measured. There are several levels of theory at which ${\displaystyle \epsilon(\mathbf q,\omega)}$ can be computed, which are described bellow.

Non-interacting electrons and holes

At this level there is no interaction between electrons and holes after the former are promoted to higher energy levels. The dielectric function is computed from the dipolar moments using

${\displaystyle \epsilon(\mathbf q,\omega) = 1 + \frac{4\pi^2 e^2}{\Omega}\frac{1}{q^2}\lim_{\delta\to 0^+}\sum_{c,v,\mathbf k}\frac{|\langle c\mathbf k |\mathbf r| v\mathbf{k-q}\rangle|^2}{\omega - \varepsilon_{c\mathbf k} + \varepsilon_{v\mathbf k - \mathbf q} + \mathrm i \delta}. }$

Here, ${\displaystyle c}$ and ${\displaystyle v}$ are electron and hole indices, respectively, and ${\displaystyle \varepsilon_{c\mathbf k}}$ and ${\displaystyle \varepsilon_{v\mathbf k - \mathbf q}}$ are their respective energies. These can come from either DFT, Hartree-Fock (HF), or GW.

In order to obtain the dielectric function at this level, the INCAR file must have the line LOPTICS=.TRUE. during a DFT or GW run. However, since the electrons and holes are taken as being non-interacting, effects such as the formation of excitons cannot be included.

Random Phase approximation (RPA)

Bethe-Salpeter equation (BSE)