Fix npt with few fixed atoms

Dear lammps users,

My system consist of chloroform liquid orcinol solid and few wall atoms (which will be immobile during simulation). The wall atoms wont interact among them and with solid, it interacts only with fluid atoms (chloroform). when i tried to do the npt simulation box volume is getting exploded after 1ns. I am attaching the input file can any one suggest me what is going wrong in my simulation.

The atom types 1-2 are for chloroform, 3-4 are for wall atoms and 5-11 are for the solid atoms. I need to keep wall atoms fixed and center of mass of solid to be at its initial value.

I have gone through the series of posts regarding similar issue in 2011 and I followed the comments given by Andrew. Still i have some issue with npt simulation.

Thanks in advance

Research Scholar

Chemical Engineering

IISC Bangalore

1.eps (56.7 KB)

input.txt (5.85 KB)

There is not enough information here to give some meaningful specific
advice. in particular, the data file is missing, so it is impossible
to consider the geometry you are trying to simulate.
i don't think, that the strategy of using a fix rigid to immobilize
atoms is a good idea. just excluding them from time integration should
be sufficient.

since this is a rather complex input, it is difficult to predict the
impact of the various settings, but the first thing, that i would
recommend you do is to simulate the system with constant volume, i.e.
fix nvt instead of fix npt and monitor, if there is any unusual
behavior and unexpected changes in potential energy or pressure. it
should simply be, that your (rather complex) force field does not
describe your system sufficiently well. i would also avoid using
r-RESPA until you have verified, that you can do a stable simulation
with verlet und have verified, that the individual time step choices
for the different levels are adequate (and then the question remains,
why not use a 2x or 3x multiplier on kspace?).


Dear axel,

thanks for your comments… I will test and check with your inputs…

meanwhile, please find the data and input files in the attachment.

I have tested two cases i) solvent with wall atoms (no solid) and ii) solvent+solid (no wall atoms) in both cases npt simulation is stable. I don’t find any meaningful reason for the abnormal behavior for the combined case.

If it is problem due to the virial contribution from immobile wall atoms, same thing should i observe in case i also.

Research Scholar

Chemical Engineering

IISC Bangalore.

npt_problem.tar.gz (457 KB)