Optical

[bosonie]

Support for optical calculations

[bosonie]

From my side, this is ready to be merged @albgar

[albgar]

It is good. Just minor comments:

• Most examples use files from the 'fixtures' directory, except workflows/example_epsilon.py and plugins/siesta/example_kgridcutoff.py.

• In the documentation for the 'epsilon' workchain, (and maybe in other files) it is written:

The **EpsilonWorkChain** is a simple extension of the **SietaBaseWorkChain** 
that introduces a post-processing step to obtain the low frequency dielectric
constant..

In fact, it is the electronic contribution to the static dielectric constant. (For polar systems there is an extra lattice contribution)

• In the same file: inputted should be input.

• In utils/protocols_system/input_generators.py, the default optical mesh is built from mesh[0] only, instead of with the possibly distinct 0, 1, and 2 components.

• Why is the epsilon output not required in the epsilon.py workflow?

  spec.output('epsilon', valid_type=orm.Float, help='Low-frequency dielectric constant', required=False)

[bosonie]

Thanks @albgar, I fixed the example and the documentation you reported.

The example_kgridcutoff.py is not part of the package, probably in your local repo? In any case, great you spot the error in the other example. This is the risk to have several PRs open, I changed systematically all the examples in the refactoring but this one was on a different branch!

Regarding the utils/protocols_system/input_generators.py the syntax you see is correct since get_kpoints_mesh return a 2x3 array where the first line is the mesh and the second is the displacements.

The epsilon output is not required due to a bad design of the BaseRestartWorkChain I believe. I do not remember exactly but I'll look into it.

For now I marge, so we go on.