H4 hydrogen-bond and X halogen-bond corrections
===============================================

The H4 hydrogen-bond correction (src/corrections/h4.rs) and the X halogen-bond
correction (src/corrections/hx.rs), including their parameters, originate from:

  J. Řezáč, P. Hobza, "Advanced Corrections of Hydrogen Bonding and Dispersion
  for Semiempirical Quantum Mechanical Methods", J. Chem. Theory Comput. 8,
  141 (2012).   [H4]

  J. Řezáč, P. Hobza, "A halogen-bonding correction for the semiempirical PM6
  method", Chem. Phys. Lett. 506, 286 (2011).   [X]

They were ported from MOPAC v23.2.5 (github.com/openmopac/mopac, tag v23.2.5,
Apache-2.0), whose src/corrections/{H_bonds4.F90, disp_DnX.F90} were "derived
and adapted from the h_bonds4 library" created by Jan Řezáč (see MOPAC's
src/corrections/README.md).

Note: pm6-rs's H4 currently omits the water- and charged-group scaling factors
of the full model; see the module documentation in src/corrections/h4.rs.
