Modeling the interactions between polyoxometalates and their environment.

To develop a force field suitable both for polyoxometalates (POMs) and organic cations, the Merck molecular force field 94x (MMFF94x) has been selected to describe the counterions used in POMs synthesis and has been combined with our force field optimized for type-II POMs with electrostatic and Van der Waals interactions included in the potential. Nontransferability of force fields is well-known and, to overcome this limitation, a charge-scaling factor (SF) has been introduced and optimized to tune the POMs force field parameters and adapt them to MMFF94x. The mixed MMFF94x/POMFF-II force field has been optimized and tested on different clusters based on hepta-molybdate. To validate our mixed force field comparison of the results obtained after molecular mechanics (MM), geometry optimizations with density-functional (DFT) calculations have been performed on the smallest system of interest. This has enabled a study of the accuracy of different functionals, especially on the description of hydrogen bonding, to be made. Results are promising in terms of structural accuracy. MM geometry optimization can be used on small POM clusters, competing reasonably well with DFT. When quantum approaches increase considerably the computational cost because of the size of the system studied, MM can be used, with the small reservation that even if the charge SF introduced improves the performance of the force field, further optimizations of the nonbonded term and the model used for the atomic charges may be necessary in further studies.