1.14*CM1A-LBCC: Localized Bond-Charge Corrected CM1A Charges for Condensed-Phase Simulations

The quality of the 1.14*CM1A and
1.20*CM5 charge models was evaluated
for calculations of free energies of hydration. For a set of 426 neutral
molecules, 1.14*CM1A and 1.20*CM5 yield MADs of 1.26 and 1.21 kcal/mol,
respectively. The 1.14*CM1A charges, which can be readily obtained
for large systems, exhibit large deviations only for a subset of functional
groups. The results for these cases were systematically improved using
localized bond-charge corrections (LBCC) by which offsetting adjustments
are made to the partial charges for atoms in specified bond types.
Only 19 LBCCs were needed to yield 1.14*CM1A-LBCC charges that reduce
the errors for the 426 ΔGhyd values to only 0.61 kcal/mol. The modified charge method
was also tested in computation of heats of vaporization and densities
for pure organic liquids, yielding average errors of 1.40 kcal/mol
and 0.024 g/cm3, similar to those for 1.14*CM1A.