Probing the Importance of Charge Flux in Force Field Modeling

We analyze the conformational
dependence of atomic charges and
molecular dipole moments for a selection of ∼900 conformations
of peptide models of the 20 neutral amino acids. Based on a set of
reference density functional theory calculations, we partition the
changes into effects due to changes in bond distances, bond angles,
and torsional angles and into geometry and charge flux contributions.
This allows an assessment of the limitations of fixed charge force
fields and indications for how to design improved force fields. The
torsional degrees of freedom are the main contribution to conformational
changes of atomic charges and molecular dipole moments, but indirect
effects due to change in bond distances and angles account for ∼25%
of the variation. Charge flux effects dominate for changes in bond
distances and are also the main component of the variation in bond
angles, while they are ∼25% compared to the geometry variations
for torsional degrees of freedom. The geometry and charge flux contributions
to some extent produce compensating effects.