Hydrated structure of ammonia–water molecule pair via the free energy gradient method: Realization of zero gradient and force balance on free energy surfaces

Abstract

The hydrated structure of ammonia molecule in aqueous solution was theoretically optimized as an ammonia–water molecule pair (H3N⋯H2O) by the free energy gradient (FEG) method [J. Chem. Phys. 113, 3516 (2000)]. The interaction between the pair and a solvent water molecule (TIP3P) [J. Chem. Phys. 79, 926 (1983)] was described by a hybrid quantum mechanical and molecular mechanical method combined with a semiempirical molecular orbital method at the PM3 level of theory. It is concluded that the present FEG method works quite well in spite of a simple steepest descent optimization scheme equipped with the adaptive displacement vector. The free energy stabilization was estimated −0.3 kcal/mol from the free energy for the same structure as that of the cluster in the gas phase. The optimized structure was found to be almost the same as that in the gas phase except for a longer OH bond length of the water molecule. However, its realization in aqueous solution is accomplished by virtue of the fulfillment of both “zero gradient” and “force balance” conditions. Finally, we discuss also the effect of microscopic “solvation entropy,” compared with the result by the conductorlike screening model method.