OPTIMIZATION OF THE CAVITY SIZE FOR AB-INITIO MST-SCRF CALCULATIONS OF MONOVALENT IONS

We present a study on the determination of the optimum solute cavity t o be used for simulating the solute/solvent interface in ab initio sel f-consistent reaction field (SCRF) calculations of charged molecules. The SCRF continuum model proposed by Miertus, Scrocco and Tomasi is co nsidered. Semiempirical and ab initio gas phase optimizations, and Mon te Carlo simulations of systems in aqueous solution suggest that the f irst hydration shell approaches more to a charged molecule than to a n eutral species. Comparison between 6-31 + + G/MST and experimental fr ee energies of hydration suggests that the optimum cavity for charged molecules is placed at around 1.10-1.15 times the van der Waals radii, which implies a reduction of almost-equal-to 0.1 angstrom with respec t to the optimum cavity size for neutral molecules.