Chloromethanes have broad applications in the industry, as they are involved in the production of refrigerants, foaming agents, aerosol propellants, and special solvents; among these, monochloromethane is extensively used for silicone production. However, like other halogenated hydrocarbons, these chemicals are considered to be atmosphere and water contaminants, which justifies the importance of studying their reactions in water. Here we studied the bimolecular nucleophilic substitution reaction of chloromethane and sodium fluoride in water. Previous studies on the matter have been realized both experimentally and theoretically , providing a benchmark to test the PBC adapted QM/MM method presented in this study. From experimental values, it is known that the activation barrier of this reaction in water is 26.9 kcal•mol -1 . Theoretical calculations have been able to closely reproduce this value, yielding results of 23.2 and 28.9 kcal•mol -1 using CCSD(T)/MM simulations. In contrast, to the best of our knowledge the reaction free energy is available only from theoretical studies, and it varies depending on the method and level of theory used in the calculations, including values around -8.0 kcal•mol -1 for CCSD(T)/MM, and -23.42 kcal•mol -1 using MP2/MM, predicting thus a more stable fluoromethane/chloride complex.