Rydberg atom van der Waals interaction, C6 calulations - angular channel precalculated values

Precalculated angular channel values to compute the C6 interaction coefficients for vdW interaction between Rydberg atoms.

The data can be utilised via the Alkali Rydberg Calculator (ARC), usage examples can be found on ARCs GitHub page, or here: https://github.com/KarenWadenpfuhl/ARC-Alkali-Rydberg-Calculator/blob/master/doc/ARC_angularChannels.ipynb

The physics underlying the angular channel approach is outlined here: Unravelling the Structures in the van der Waals Interactions of Alkali Rydberg Atoms.

Data type is HDF5 with associated metadata.

We provide data for Rb-Rb and Cs-Cs pair states with n1, n2 in range 20-120 and angular quantum numbers L in {S, P, D, F}. Other atomic species combinations, also cross-species interactions like Rb-Cs, or for different principla quantum numbers n1, n2, or for different angular quantum numbers L1, L2, can be computed by the user. Appropriate functions are provided via ARC. An example Jupyter notebook detailling how to download the precalculated data, and generally how to use the angular channel functions, can be found here: https://github.com/KarenWadenpfuhl/ARC-Alkali-Rydberg-Calculator/blob/master/doc/ARC_angularChannels.ipynb