Evaluating the Effects of Geometry and Charge Flux in Force Field Modeling

We apply a model
for analyzing the importance of conformational
charge flux to 11 molecules with the R–(CH2)n–R structure (R = Cl, F, OH, SH, COOH,
CONH2, and NH2 and n = 4–6).
Atomic charges were obtained by fitting to results from density functional
theory calculations using the HLY procedure, and their geometry dependence
is decomposed into contributions from changes in bond lengths, bond
angles, and torsional angles. The torsional degrees of freedom are
the main contribution to the conformational dependence of atomic charges
and molecular dipole moments, but indirect effects due to changes
in bond distances and angles account for ∼15% of the variations.
While the magnitude of charge flux and geometry effects have been
found to be independent of the number of internal degrees of freedom,
the nature of the R- group has a moderate influence. The indirect
effects are comparable for all of the R-groups and are approximately
one-half the magnitude of the corresponding effects in peptide models.
However, the magnitudes are different, yet the relative importance
of geometry and charge flux effects are completely similar to those
of the peptide models, which suggests that modeling the charge flux
effects for changes in bond lengths, bond angles, and torsional angles
should be considered for developing improved force fields.