A new treatment is proposed for application to a solute-solvent system of adaptive multiscale QM/MM-MD method that enables the solvent molecules to flow across the boundary between the QM and MM regions by introducing the transition (T) one. In contrast to previous treatments, we define both the QM and T regions by using a number of solvent molecules around the solute, instead of the distance from it. For demonstration, we have applied it to pure water; it has shown that the present method can work effectively with a shorter computation time linearly-scaled by the number of QM calculations. (C) 2011 Elsevier B. V. All rights reserved.