Question about the union of two regions

Hi All,

I have a question related to the union of two region

In my simulation, I am currently defining a region as a sum of two adjacent cubes.
By looking at the .vtk of my simulation in paraview, I see that there is a small "empty space" (or gap) between the regions and I don't understand if

1) that comes from the division of the region (although it supposed to look like a single piece)
region regdown block 0.0 0.5 0.0 0.5 0.0 0.25 units box
region regup block 0.0 0.5 0.0 0.5 0.25 0.5 units box
region reg union 2 regdown regup units box

2) or from the distribution of particles
fix ins nve_group insert/pack seed 5330 distributiontemplate pdd2 &
maxattempt 100 insert_every once overlapcheck yes all_in yes vel constant 0. 0. 0. &
region regdown volumefraction_region ${alphastartporous}

fix ins2 nve_group2 insert/pack seed 5330 distributiontemplate pdd1 &
maxattempt 100 insert_every once overlapcheck yes all_in yes vel constant 0. 0. 0. &
region regup volumefraction_region ${alphastartporous}
run 1000

3) paraview just does it!

my code is attached together with a snapshot of the simulation on paraview

Hoping someone could clarify my doubts

Greetings,

Luca