Some problems with using npt in reax

Dear lamps-users,
I want to simulate the thermal decomposition products of epoxy resin under constant temperature and constant pressure environment.

I want to use the fix npt. But I received an warning that “WARNING: Fix qeq/reax CG convergence failed after 200 iterations at 225 step (…/fix_qeq_reax.cpp:734)”, then the simulation is terminated.

here is my input:

units real
dimension 3
boundary p p p
atom_style charge
read_data data.DGEBAa

pair_style reax/c control.reax_c.rdx
pair_coeff * * HCONSB.ff C H O N S

neighbor 2.0 bin
neigh_modify every 10 delay 0 check no

fix 1 all reax/c/species 1 100 100 species.out cutoff 3 3 0.6 element C H O N S position 1000 AuOH.pos

timestep 0.20
fix 2 all qeq/reax 1 0.0 10.0 1.0e-6 reax/c
minimize 1.0e-4 1.0e-6 10000 100000

fix 3 all box/relax iso 1.0 vmax 0.001

run 20000
unfix 3

velocity all create 0.01 487639
fix 4 all npt temp 0.1 0.1 20 iso 1.0 1.0 20

run 15000

When you run to this “run”, a warning will appear and the simulation will stop running.

How should I use “fix npt” in reaxc? How to avoid this error.

Looking forward to your suggestions.
Thank you.

J K Chi

due to lack of data and force field file and output log, it is impossible to reproduce or comment specifically what your problem is.

also, please always report which version of LAMMPS you are using.

the warning you mention is an indication that something is going wrong with the structure. that can have many causes. one thing to try is to equilibrate your system first at constant volume with fix nvt instead of fix npt.

please also note, that there appears to be some mixup in your input.
you are defining fix box/relax after the minimization and then “run” an MD without having defined a time integration fix, but with fix box/relax enabled. that makes no sense and LAMMPS should print a warning about that, too.