Microscopic hydration mechanism in the ammonia dissolution process: Importance of the solute QM polarization

Abstract

Abstract Using a hybrid QM/MM-MD method, the hydration process of an NH 3 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 NH 3 molecule in dissolution process.