Hi LAMMPS users,
I would like to simulate a system of rod-coil copolymers. With Axel,s help, I can manage to simulate the system with NPT (iso). However, when I checked the output, I found the pressure is a little bit off from the target pressure by 0.5 (LJ unit) in average consistently. In my simulation, I have used fix rigid/nvt for rod parts and fix npt dilate partial for coil parts. Could any one tell me why the pressure is off from the target pressure? Thank you in advance.
Second question: Since I expect the lamellae formation, I would like to do NPT simulations to allow dimensions to change independently with fix npt tri option. I set up my simulation in equilibration mode and production mode sequentially by:
dump 1 all atom 10000 eq.T${t2}P${pf}.lammpstrj
dump_modify 1 scale no
run 200000 ##### equilibration
undump 1
dump 1 all atom 10000 prod.T${t2}P${pf}.lammpstrj
dump_modify 1 scale no
run 300000 #######production
However, the simulation would crash after the equilibration. The error message is" ERROR: Fix rigid: Bad principal moments". Then I try to extend the equilibration time, it works fine til the production mode. I checked the source code and found that LAMMPS might check the principal moments by init() in fix_rigid.cpp when starting to run the production. If it is important to check the principal moments, I am wondering why it is checked only once by init(). If the principal moments go bad during simulations, should I worry about it?
PS: if anyone would like to debug it, I have prepared all files and am more than happy to give it to you.
Typo: at http://lammps.sandia.gov/doc/fix_rigid.html
keyword = temp or press or tparam or pparam or force or torque
*press* values = Pstart Pstop Pperiod
Pstart,Pstop = desired temperature at start/stop of run (pressure units)
Pdamp = pressure damping parameter (time units)
*pparam* values = Pchain
Pchain = length of Nose/Hoover barostat chain
I think the above info used only for barostat, so it is not correct for fix rigid.
Third question: When I used fix npt tri, the system might crash because “fix npt/nph has tilted box beyond 45 degrees”. The document had mentioned this problem and pointed out three possible error source:
The three most common sources of this error are using keyword tri on a liquid system, specifying an external shear stress tensor that exceeds the yield stress of the solid, and specifying values of Pstart that are too far from the current stress value.
In my simulation, I had equilibrated the system at P=30 and T=3.0 by fix npt iso, then I quenched the system to T=0.7 by velocity scale and kept P=30 by fix npt tri. I checked the instantaneous press after velocity scale is about 26. I don’t know if the system crash due to large difference of Pstart and Pstop. Could anyone give me some suggestion how to prevent the crash? Thanks.
Best,
Dongsheng