**Short description**

Provide a clear and concise description of what is (to be) done in this pull request.

**Related issues**

Link to the open (and possibly closed) issues that are related to this pull request. If this pull request fully closes an issue, please use GitHub's closing keywords. 

**To do**

- [ ] (if applicable, when creating new source files): don't forget to update the Doxygen file, the collective `gqcp.hpp` include header and the CMake files
- [ ] Add other smaller task to be completed as a Markdown task list
