Describe the bug
D4_model calculator sometimes returns NaNs from get_forces()

To Reproduce
To reproduce with dftd4:

1. Unpack the archive: POSCAR.tar.gz
2. Run
   dftd4 --func pbe --grad hcp.POSCAR
3. Check that gradient file contains NaNs

Whilst
dftd4 --func pbe --grad fcc.POSCAR
works well

To reproduce with Python:

1. Unpack the archive: dftd4-get_forces.tar.gz
2. Run test.py
3. Check that for hcp.py the test prints nan's