Role of immobile rigid atoms in pressure calculations

I need little clarification on this thread about NPT fix in system
with solvent and immobile objects :

I quote
"what you _really_ want is to have the rigid object particles moved
_with_ the rest, but only as a center of mass motions."

Does it mean I should also time integrate my rigid immobile object?


I am trying to simulate a system with infinitely periodic sheets, held
constant by partial dilation (excluding sheet from integration).
Hence I can only give NPT with z 1.0 1.0 1000, but as discussed there
it gives pressure of 6000 atm and really wrong volume.

I took one of the suggestion from that thread and plotted pressure vs
volume graph (changing volume in z direction with fix deform), and
fitting it to virial of form p = a/v + b/v^2 + c/v^3
following by estimation of volume at which i will get 1 atm average
pressure and run it now as NVT. As of now it looks like it is working

However I noticed that if i include fix rigid in it , by making the
immobile sheet rigid, it gives different pressure/volume.
Which is quite close to what I calculated through isotherm. I could
only attribute it to this statement

"ignoring the forces (and stress tensor contributions) from the
interactions inside the rigid body is a must (they are ignored
by the rigid integrator, too)"

but now I am little confused whether I should also time integrate my
rigid sheet? Or just giving fix rigid is enough? Or should i do both
manual calculation with fix rigid?
Also I cannot give neigh_modify exclude as suggested because am
running it on GPU and apparently it is not allowed.

Also as a favor, is there any good textbook recommendation on MD of
rigid objects? Frenkel and Smith, and leach were little sketchy in
that region
LAMMPS ver 27 Aug 2016 ICMS branch