A m ethod of combined quantum mechanical and molecular mechanical (QM/MM) calculations is suggested and applied on MD simulations of water systems using combined QM/MM potentials. In the MD simulations, the QM calculations are carried out at the ab initio Hartree-Fock level and TIP3P water models are used to represent the MM water molecules. We have focused on the combined QM/MM coupling potentials. Our results show that the QM/MM coupling is very sensitive to the Lennard-Jones parameters used on the QM atoms. When the TIP3P Lennard-Jones parameters are used for the QM atoms, the QM/MM coupling is found to be too strong. However, when the Lennard-Jones parameters on the QM atoms are optimized according to the corresponding QM/MM calculations of water dimer (dimer optimized parameters, DOP), the QM/MM coupling seems to be too weak. Therefore, we believe that the Lennard-Jones parameters on the QM atoms optimized from the corresponding QM/MM calculations for some molecular complexes cannot reliably be used in the simulation of the condensed phases. A modified QM/MM coupling for water system is found by scaling down the MM charges appearing in the coupling potential. Studies show that the new QM/MM coupling potentials can be used satisfactorily in the combined Hartree-Fock QM/MM molecular simulations of water from ambient to supercritical conditions.

References

Yaoquan Tu and Aatto Laaksonen
On the effect of Lennard-Jones parameters on the quantum mechanical and molecular mechanical coupling in a hybrid molecular dynamics simulation of liquid water.
Journal of Chemical Physics, 111(16), 7519-7525 (1999).

Yaoquan Tu and Aatto Laaksonen
Combined Hartree-Fock quantum mechanical and molecular mechanical molecular dynamics simulations of water at ambient and supercritical conditions.
Journal of Chemical Physics, 113(24), 11264-11269 (2000).