  A straightforward means to include explicit hydrogen bonds within the
Universal Force Field is presented. Instead of treating hydrogen bonds as
non-bonded interaction subjected to electrostatic and Lennard-Jones potentials,
we introduce an explicit bond with negligible bond order, thus maintaining the
structural integrity of the H-bonded complexes and avoiding the necessity to
assign arbitrary charges to the system. The explicit hydrogen bond changes the
coordination number of the acceptor site and the approach is thus most suitable
for systems with under-coordinated atoms, such as many metal-organic
frameworks, however, it also shows excellent performance for other systems
involving a hydrogen-bonded framework. In particular, it is an excellent means
for creating starting structures for molecular dynamics and for investigations
employing more sophisticated methods.
  The approach is validated for the hydrogen bonded complexes in the S22
dataset and then employed for a set of metal-organic frameworks from the
Computation-Ready Experimental (CoRE) database and several hydrogen bonded
crystals including water ice and clathrates. We show that direct inclusion of
hydrogen bonds reduces the maximum error in predicted cell parameters from 66%
to only 14% and the mean unsigned error is similarly reduced from 14% to only
4%. We posit that with the inclusion of hydrogen bonding, the solvent-mediated
breathing of frameworks such as MIL-53 is now accessible to rapid UFF
calculations, which will further the aim of rapid computational scanning of
metal-organic frameworks while providing better starting points for electronic
structure calculations.
