D3 and D4 empirical dispersion corrections

[kjelljorner]

I could not get the install of the dftd3 extension to work (MacOS). My suggestion would be to link to the new dftd4 package which is installable by conda. There is also a Python interface that makes calculation of dispersion energies trivial. This is also installable via conda. I know that @awvwgk also has plans to make his D3 calculator available via this route as well.

[awvwgk]

Working on native support for PySCF in https://github.com/awvwgk/simple-dftd3/pull/17 now. The dftd3.pyscf module should be a drop-in replacement for pyscf.dftd3.itrf, except for the DFT-D2 method which is missing in my implementation.

[sunqm]

The pyscf interfaces provided by dftd3 https://dftd3.readthedocs.io/en/latest/api/pyscf.html and dftd4 https://dftd4.readthedocs.io/en/latest/reference/pyscf.html are the recommended way to add dispersion corrections.