fix gcmc + two parallel walls


I am trying now to use fix gcmc to study the adsorption of water between two parallel walls (defined with fix wall). I am using a simulation cubic simulation box, with edge limits 0 and 20. I also define a region so that water molecules are only inserted between the walls (which are located at the planes z=0 and z=20). However I am not sure that I am doing correctly because when I set the region as :

region slab block 0. 20. 0. 20. 0.8 19.2 side in units box

the simulation hangs after 1100 steps (it does not exit, but it does not continue either). Again it seems that the code has entered an infinite loop.
I tried without “side in” but the result is the same.

Only when I do:

region slab block 0.8 19.2 0.8 19.2 0.8 19.2 side in units box

the code exits without error, i.e., in spite of the fact that I have periodic boundary conditions in x and in y, I have to reduce the region with a slice of the order of the size of the water molecule, so that when you insert one molecule one of the atoms does not lie outside the simulation box (even though my box is periodic in x and y directions)

However, I do not think that this is correct, as this could be introducing a bias in the GCMC. I think insertions/deletion attempts should occur with the same probability in all the simulated region (and we are not doing so in a slice in x and y directions). Although this might corrected by the NVT MD simulations, not sure…

Can this be fixed in some way I am missing? Does my input file seem correct for the purpose explained?

I send attached the input files.

Thank you very much in advance for your help.

Best wishes

h2o.txt (675 Bytes)

in.gcmc.tip4p2005 (3.38 KB)