geometry optimisation with fixed dihedral

[xiki-tempula]

Is your feature request related to a problem? Please describe.

I'm interested in integrating the xTB into a package for the automatic procedure and it would be good if one could do a geometry optimisation with fixed dihedral using the xTB python interface.

This is an extension of #38

Describe the solution you'd like

The interface could mimic the openMM interface

[awvwgk]

Requires https://github.com/grimme-lab/xtb-python/issues/54, https://github.com/grimme-lab/xtb-python/issues/38

[awvwgk]

Somebody has to come up with a stable API we can implement in C to drive geometry optimizations externally (this has to happen in xtb).

[xiki-tempula]

@awvwgk Do you mean a C API or a python API?

[awvwgk]

We have to implement it in a C API first.