Fast and accurate calculation of hydration energies of molecules and ions.

We present an efficient method with adjustable parameters for calculating the hydration free energy of molecules and ions using the gas-phase geometry and atomic charges. In most cases, the method yields accurate results, with a mean absolute error for neutral molecules below 1 kcal mol-1 and for ions about 3 kcal mol-1. Overall, despite its simplicity, the proposed method shows the best performance among major computational approaches applied to estimate hydration free energies. The method requires as input Cartesian cordinates and CM5 atomic charges only, which are easily available from any DFT calculation, and yields the hydration energy in a matter of seconds for a medium-size molecule or ion.