# [lammps-users] pressure when there is a rigid region in the system

Hi Steve,

This is related to a question Ly Le asked about a week ago, and you responded but I am not sure I understand.

I want to do some simulations in which part of the system is kept rigid, and part is mobile. I started with a simple
case in which I make rigid a portion (“wall”) of an atomic system, and compared some thermo output for that “wall+mobile”
case compare with the “all mobile” case (run for 0 steps, 12Feb version). The results are understandable for T and energy,
but very odd for the pressure:

step temp pe etotal press pxx pyy pzz
all mobile 0 300 -5390.1874 -4933.2328 171.97493 -460.29472 422.94152 553.278
wall+mobile 0 279.3853 -5390.1874 -5195.3134 -22262.25 102897 4649.3093 -174333.06

If Lammps computes the pressure according to:

What is the N used for the case in which part of the system is rigid? Is the difference observed reflecting only the kinetic term?
What would be the best way to compute in lammps the pressure of the mobile subset (including their interaction with the rigid)?
If I use a fix npt on the mobile, what p the program will use to expand/contract the volume?

thanks for your help,
Vale

below is the relevant part of the script. The system is periodic. The regions, groups and fixes I use are here:

region fixed block .0 10.0 INF INF INF INF units box
group wall region fixed
group mobile subtract all wall

velocity all create \${Tinitial} 2148459 rot yes mom yes dist gaussian

#in the all mobile I use only one fix for all:
fix 2 all nvt {Tinitial} {Tfinal} 500.0

– OR –

# in the wall+mobile case:

fix 1 wall rigid single
fix 2 mobile nvt {Tinitial} {Tfinal} 500.0

# and then for both i output:

thermo_style custom step temp pe etotal press pxx pyy pzz
thermo 100