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Illustration of different boundary conditions: a) parallelepiped with periodic boundary conditions (pbc:s) in all three directions, b) parallelepiped with pbc:s in the x and y directions, c) parallelepiped with pbc in the z direction, d) cylindrical boundary conditions, e) ellipsoidal boundary conditions, and f) spherical boundary condition. The gray areas represent impenetrable surfaces. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
