  We perform molecular dynamics simulations to investigate the diffusive motion
of TIP5P water in the direction perpendicular to the two hydrophobic confining
walls. To calculate the diffusion coefficient, we use the concept of the
characteristic residence time which is calculated from the exponential decay of
the residence time probability distribution function. We find that a diffusion
anomaly of water, increase of diffusion upon compression, is absent in the
direction perpendicular to the confining walls down to the lowest temperature
we simulate, 220K, whereas there is a diffusion anomaly, similar to that in
bulk water, in the direction parallel to the walls. The absence of a diffusion
anomaly may arise mainly due to nanoconfinement, rather than due to the
hydrophobic property of the confining walls.
