# Toward a Physically Motivated Force Field: Hydrogen Bond Directionality from a Symmetry-Adapted Perturbation Theory Perspective

journal contribution

posted on 28.01.2016, 00:00 by Maxim Tafipolsky, Kay AnsorgIt is argued here that the functional
forms adopted in almost all
popular force fields are too restrictive to allow for accurate and
physics-based parametrization. Some important modifications are suggested
based on symmetry-adapted intermolecular perturbation theory, which
directly separates the intermolecular interaction energy into four
physically interpretable components: electrostatics, exchange-repulsion,
dispersion, and induction. The exact electrostatic energy is approximated
as a sum of the short-range contribution (due to charge density penetration
effects), included explicitly, and the long-range part (via distributed
atomic multipoles), whereas the induction energy is evaluated by means
of the distributed induced damped point dipole model. The dispersion
energy is fitted to a simple analytical function and the exchange-repulsion
contribution is approximated by the overlap of the valence-only electron
charge densities of monomers. The water dimer is used to illustrate
the approach and to discuss its potential and possible improvements.
Analysis of the four main contributions to the binding energy allows
for a deeper understanding of the hydrogen bond directionality. It
is found that a notorious geometrical preference in the water dimer
results mainly from large polarization contributions, including induction
and dispersion.