THE SOLVATION OF SODIUM-IONS IN WATER CLUSTERS - INTERMOLECULAR POTENTIALS FOR NA-H2O AND H2O-H2O()

New model potentials are constructed for Na+-H2O and H2O-H2O, using qu antum mechanical calculations of the monomer wavefunctions. One parame ter in each model potential is fitted to experimental dimer properties . The resulting Na+-H2O potential is consistent with published thermod ynamic and ab initio data, and the H2O-H2O potential reproduces the st ructure and energy of the water dimer at equilibrium, and the second v irial coefficient of steam, within experimental uncertainties. The don or-acceptor interchange tunnelling pathway on the water dimer potentia l energy surface has a lower energy barrier than the acceptor-acceptor interchange, in agreement with recent spectroscopic studies. When a s imple non-additive induction potential is included, calculated thermod ynamic properties of solvated sodium ions are in agreement with experi mental data. For small clusters in the gas phase, the first solvation shell of the sodium ion is predicted to contain four water molecules.