fix rigid , npt/rigid , rigid/nvt , ...

Hi LAMMPS users ,

I have a system with one C60 inside of about 1000 water molecules .
I want to run my simulation in the NPT ensemble and also I want to consider C60 as a rigid molecule .

first , I used this commands

fix 1 SPC shake 1.0e-4 500 1000 a 3 b 2
velocity all create 1.0 12345689 dist uniform

fix 2 C60 rigid/npt molecule temp 300.0 300.0 100.0 iso 1.0 1.0 1000.0
fix 3 SPC npt temp 300.0 300.0 100.0 iso 1.0 1.0 1000.0

with using of this command I could control pressure at around 1 atm , but later I understood this is error because there can only be one fix which monitors the global pressure and changes the simulation box dimensions , I also got an error " Non-numeric box dimensions - simulation unstable " after changing my LAMMPS version !

now I am using this command :

fix 1 SPC shake …
velocity all …
fix 2 SPC npt temp 300.0 300.0 100.0 iso 1.0 1.0 1000.0 dilate all
fix 3 C60 rigid/nvt molecule temp 300.0 300.0 100.0

and also I check this one :

fix 1 SPC shake …
velocity all …
fix 2 C60 rigid molecule
fix 3 SPC npt tem 300.0 300.0 100.0 iso 1.0 1.0 1000.0

At these two command Pressure has great fluctuation and actually I can not understand which one is suitable for my simulation ,I will be grateful if you can help me .
Do i need more study ? !!

Thanks for your attention .
Best ,
Saeed.

The fix rigid doc page discusses different options for hybrid systems.

The sub-section starts with this sentence:

If your simlulation is a hybrid model with a mixture of rigid bodies and non-rigid particles (e.g. solvent) there are several ways these rigid fixes can be used in tandem with fix nve, fix nvt, fix npt, and fix nph.

Steve