The only place you use VarMin and VarMax is the line
particle(i).Position = unifrnd(VarMin, VarMax, VarSize);
to initialize the values.
However, as the particle moves, the velocity could easily have it go below VarMin or above VarMax, since you do not catch that it hits those boundaries.
You need to program in a test for hitting the boundaries, and you need to decide what you want to do with that case.
