Pl. Cummins et Je. Gready, Coupled semiempirical quantum mechanics and molecular mechanics (QM/MM) calculations on the aqueous solvation free energies of ionized molecules, J COMPUT CH, 20(10), 1999, pp. 1028-1038
The aqueous solvation free energies of ionized molecules were computed usin
g a coupled quantum mechanical and molecular mechanical (QM/MM) model based
on the AM1, MNDO, and PM3 semiempirical molecular orbital methods for the
solute molecule and the TIP3P molecular mechanics model for Liquid water. T
he present work is an extension of our model for neutral solutes where we a
ssumed that the total free energy is the sum of components derived from the
electrostatic/polarization terms in the Hamiltonian plus an empirical "non
polar" term. The electrostatic/polarization contributions to the solvation
free energies were computed using molecular dynamics (MD) simulation and th
ermodynamic integration techniques, while the nonpolar contributions were t
aken from the literature. The contribution to the electrostatic/polarizatio
n component of the free energy due to nonbonded interactions outside the cu
toff radii used in the MD simulations was approximated by a Born solvation
term. The experimental free energies were reproduced satisfactorily using v
ariational parameters from the vdW terms as in the original model, in addit
ion to a parameter from the one-electron integral terms. The new one-electr
on parameter was required to account for the shortrange effects of overlapp
ing atomic charge densities. The radial distribution functions obtained fro
m the MD simulations showed the expected H-bonded structures between the io
nized solute molecule and solvent molecules. We also obtained satisfactory
results by neglecting both the empirical nonpolar term and the electronic p
olarization of the solute, i.e., by implementing a nonpolarization model. (
C) 1999 John Wiley & Sons, inc.