On the hardness evaluation in solvent for neutral and charged systems

The influence of water on the hardness values of a series of neutral and charged molecules has been studied in the framework of density functional theory using the polarizable continuum model to take into account solvent effects. Three working formulas already widely tested in gas-phase have been used and the results compared. Two of the methods employed going from gas phase to solvent phase give values that do not change, while the results of the third method show remarkable changes. To check the reliability of the hardness behavior found, a test based on the hard - soft/acid -base principle and the calculation of the free energy of reaction has been applied using the adopted procedures.