# 10.2. Ion-clamped dielectric tensor

In this section we consider response of materials to a static electric field perturbations. Corresponding response function is the ion-clamped dielectric constant \(\epsilon^{\infty}_{\alpha\beta}\), which characterizes electronic cloud reaction in \(\beta\) direction due to an applied electric field in \(\alpha\) direction. The electric field is assumed to have long-wavelength so that it cannot cause excitations between electronic states, and simulations are performed at clamped ions, hence the term ion-clamped.

Note

Response to a static electric field is not well-defined for metallic systems.

RESCU assumes electric fields along three Cartesian directions by default, and calculates the
full \(3\times 3\) dielectric constant. At the end of this DFPT calculation, one can also have
access to perturbed wavefunctions and self-consistent potential, which can be used in various kinds of
nonself-consistent calculations (e.g. see The Raman spectrum and non-linear optical susceptibility). If desired, direction of electric field
perturbation can be set through `dfpt.eField`

keyword, explained in more details in the following.

Here, we use the ground state data of boron nitride obtained in Preparing the ground state data. Below, a sample input file for DFPT calculation is shown:

```
info.calculationType = 'dfpt-dielectric'
info.savepath = './results/bn_real_dielectric'
rho.in = './results/bn_real_scf.mat'
psi = './results/bn_real_scf.h5'
dfpt.eField = [1 0 0;0 1 0;0 0 1] % optional
mixing.tol = 1e-5*[1 1] % advised for dfpt
option.saveWavefunction = 1 % only required if perturbed wavefunctions are supposed
% to be used for subsequent nonself-consistent calculation
```

The important keywords for this type of DFPT calculation are as follows:

`info.calculationType`

this central keyword determines the type of the DFPT calculation to perform;`info.savepath`

points to the file which stores the output data DFPT calculation;`rho.in`

this DFPT calculation loads the ground state electronic density at the beginning of the computation, and this keyword points to the file which contains the output of the ground state calculation, so that RESCU can load corresponding electronic density data;`psi`

this DFPT calculation also loads the ground state wavefunctions at the beginning of the computation, and this keyword points to the file which contains the stored ground state wavefunctions;`dfpt.eField`

the electric field directions along which the ion-clamped dielectric tensor is calculated. Three linearly independent directions are required to obtain the dielectric tensor. This keyword is optional and the default are the three Cartesian directions;`mixing.tol`

first entry sets the convergence criterion for the perturbed self-consistent density and the second entry sets the criterion for the perturbed self-consistent potential.`option.saveWavefunction`

determines whether first-order perturbed wavefunctions should be saved to disk. This is only necessary if one wishes to perform a subsequent DFPT calculation based “2n+1” theorem to compute non-linear response functions.

Warning

Storing the perturbed wavefunctions requires a large amount of disk space, and one must make sure that they are only saved if are to be used later

Once the input file is created, RESCU can be invoked by

```
rescu -i bn_real_dielectric
```

RESCU computes perturbed wavefunctions with respect to each electric field direction, from which the polarization and dielectric tensor are calculates according to [BGCG01]:

Various output information can be found in `dfpt`

field of structure found in
`./results/bn_real_dielectric.mat`

.

```
>> load ./results/bn_real_dielectric.mat
>> dfpt
dfpt =
struct with fields:
desc: [1x1 struct]
dielectricTensor: [3x3 double]
eField: [3x3 double]
polarization: [3x3 double]
qpointDirect: [0 0 0]
qpointGridn: [1 1 1]
type: 'dielectric'
```

Polarization and ion-clamped dielectric tensor data can be found as a \(3\times 3\) matrix in
`dfpt.polarization`

and `dfpt.dielectricTensor`

, respectively. In our case, we have

```
>> dfpt.polarization
ans =
0.4007 -0.0000 -0.0000
-0.0000 0.4007 -0.0000
-0.0000 -0.0000 0.4007
>> dfpt.dielectricTensor
ans =
6.0352 -0.0000 -0.0000
-0.0000 6.0352 -0.0000
-0.0000 -0.0000 6.0352
```

These quantities already provide valuable information regarding the simulated material. However, together with perturbed wavefunctions, they can also be used in post-analysis DFPT calculations to obtain further insight about response functions of materials.

Stefano Baroni, Stefano De Gironcoli, Andrea Dal Corso, and Paolo Giannozzi. Phonons and related crystal properties from density-functional perturbation theory. Reviews of Modern Physics 73.2 (2001), p. 515.