[gmx-users] what's the meaning of pbc=no?

[Dai Liang]

I want to do a simulation without periodic boundary condition, but I am confused by the meaning of pbc=no.
It may have two meanings. 
First, no box, then liquid water becomes vapour because pressure is zero.
Second, there is a box. any atom who collides the box will be reflexed. but here no image box.

The simulation I intend to do is the second one, but gromacs seems to do the first one.

who can clarify it? thanks
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