Critical Assessment of the Hybrid QM/MM-pol-vib Approach: Small Water Clusters Using Polarizable Flexible Water Potentials

We report a systematic study of the structure of small water clusters, up to pentamers, using a hybrid quantum-mechanics/molecular-mechanics approach with polarizable flexible water-interaction potentials in conjunction with HF SCF wave functions. The model is denoted QM/MM-pol-vib. For each optimized QM cluster, we replaced QM water molecules one at a time with MM-pol-vib water molecules and reoptimized the cluster structure. We found that the hybrid structures and energies reproduce well their full QM counterparts. This finding indicates that the first hydration shell of solvation obtained with such a model is described at a semiquantitative level of accuracy. The model should prove useful in modeling aqueous reactions. We outline the efficient computational strategy adopted for coupling the polarizable response of the solvent with the solute wave function calculation. Energy gradients for the solute and the solvent molecules are also efficiently calculated.