Using a hybrid QM/MM-MD method, the hydration process of an NH3 molecule from vapor to liquid phase across the liquid-vapor interface was investigated. To reduce enormous computational cost for free energy estimation, we employed for QM calculation, the NDDO-SSRP method, an optimum strategy using the semiempirical MO method. The hydration free energy was estimated as -4.5 kcal/mol, being in good agreement with -4.3 kcal/mol (the experimental value), while overestimated in full MM treatment with the point dipole model. It was concluded that the solute QM polarization should be essentially important to reproduce the polarization of NH3 molecule in dissolution process. (C) 2009 Published by Elsevier B. V.