Accurate Partial Atomic Charges for High-Energy Molecules Using Class IV Charge Models with the MIDI! Basis Set

Kelly, C. P.; Cramer, C. J.; Truhlar, D. G.

* Theor. Chem. Acc.*
**2005**, *113*, 133.

We have recently developed a new class IV charge model for calculating
partial atomic charges in molecules. The new model, called Charge Model 3
(CM3), was parameterized for calculations on molecules containing H, Li, C,
N, O, F, Si, S, P, Cl, and Br by Hartree Fock theory and by hybrid density
functional theory (HDFT) based on the modified Perdew Wang density
functional with several basis sets. In the present article, we extend CM3
for calculating partial atomic charges by Hartree Fock theory with the
economical but well balanced MIDI! basis set. Then, using a test set of
accurate dipole moments for molecules containing nitramine functional
groups (which include many high energy materials), we demonstrate the
utility of several parameters designed to improve the charges in molecules
containing both N and O atoms. We also show that one of our most recently
developed CM3 models that is designed for use with wave functions
calculated at the *m*PW*X*PW91/MIDI! level of theory (where
*X* denotes a
variable percentage of Hartree-Fock exchange) gives accurate charge
distributions in nitramines without additional parameters for N and O. To
demonstrate the reliability of partial atomic charges calculated with CM3,
we use these atomic charges to calculate polarization free energies for
several nitramines, including the commonly used explosives
1,3,5-trinitro-*s*-triazine (RDX) and
2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW), in nitromethane.
These polarization
energies are large and negative, indicating that electrostatic interactions
between the charge distribution of the molecule and the solvent make a
large contribution to the free energy of solvation of nitramines. By
extension, the same conclusion should apply to solid state condensation.
Also, in contrast to some other charge models, CM3 yields atomic charges
that are relatively insensitive to the presence of buried atoms and small
conformational changes in the molecule, as well as to the level of
treatment of electron correlation. This type of charge model should be
useful in the future development of solvation models and force fields
designed to estimate intramolecular interactions of nitramines in the
condensed phase.

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