❓ Should we also implement an electronic band structure example?

[mbercx]

The EOS workflow example already highlights various aspects of how the engines deal with certain aspects of scientific workflows:

1. Executing Python code.
2. Executing external code.
3. Connecting steps in a workflow.
4. Running multiple external calculations in parallel.
5. Waiting for long-running calculations before executing the next step.
6. Persistance.

And probably more. However, one aspect that isn't covered at the moment is restarts. I.e. what if the second calculation requires access to the full output files of the first?

A typical example here is a band structure calculation, where you would first run an SCF to obtain the charge density, and then an NSCF along a k-path. Quantum ESPRESSO loves working with restarts, typically running multiple different "codes" (it's a code suite, after all). Another example would be the DOS. For this you'd run:

pw.x (SCF) -> pw.x (NSCF) -> dos.x (post-processing)

and each restart requires (large) output files from the previous step. So I'm curious to see how other workflow engines specify parent directories, indicate that a follow-up step should wait for the previous, make sure the files are only cleaned up at the end, etc...

Another note: the next step in the verification project (after EOS) is also band structures, i.e. construct a common band structure workflow for various "quantum engines" (Quantum ESPRESSO, VASP, ABINIT, ...) and compare results.