Remove CPCM-X library

[foxtran]

This PR removes dependency of CPCM-X library.

The reasons why:

• often leads to failure of CI tests that may affect users too
• no geometry optimizations
• API is used through files

[foxtran]

@thfroitzheim, can it be merged?

[thfroitzheim]

While I see the stability problems with CPCM-X, it is an actively used feature of xtb and cannot just be removed. We are currently working on new solvation models, so maybe we can replace CPCM-X at some point in the future

[toxtran]

actively used feature of xtb

Do you have statistics about CPCM-X + xtb combination?

We are currently working on new solvation models,

There is GBSA solvation model which supports grafients, while CPCM-X does not.

[corinwagen]

Our team uses CPCM-X a lot for various applications, like pKa prediction and other soon-to-be-released work, so this would be quite damaging for us. It's very useful to have accurate low-cost solvation energies, even without gradients.

[awvwgk]

Agree that this might be the right way forward for now. If this feature will be reactivated we can bring it back again from the git history.

[awvwgk]

Corin @corinwagen, thank you highlighting this use case, in general I would like to keep features available in xtb especially if there are active users. In case of CPCM-X there is currently no maintainer available for fixing bugs or even just creating new releases, if you are able to commit some time to (co-)maintain CPCM-X support in xtb, it would make the support of this feature much more feasible.

[corinwagen]

We'd love to help but neither @jevandezande nor I know FORTRAN, sadly

[TyBalduf]

We also use CPCM-X for a few different applications at Schrodinger. I think we would be able to contribute towards maintaining this feature.

Does it make more sense to maintain this separate CPCM-X library or to reimplement CPCM-X into tblite? I may be misunderstanding, but it seems like it wouldn't be too extensive an addition to tblite once the ddX interface is added tblite/tblite#235

[lukaswittmann]

The problem with implementing this in, e.g., tblite will be, that we fixed #1159 (and #1163). This means, that CPCM-X will not work with the new implementation (i.e., the results will likely be not usable without a rework and parameterization) and it would require us to ship two different implementations of the ddCOSMO scheme.