I am studying periodic structures of small organic molecules (viz., amines, etc.). I would like to sample a given system’s high and low energy structures through MD simulations. I am trying to work with NPT with a range of temperatures (viz., 50K, 300K, 500K ) and pressures (1bar/atm, 1000 bars/atm, -1000 bars/atm). However, I observed that the volume blows up (or expands significantly) during the simulation when the pressure is low (viz., 1 bar/atm, -1000 bars/atm) and the temperature is high (viz., 500K). The high-temperature simulations run fine when the pressure is high (viz., 1000 bars/atm). I have been trying to understand the physical reason behind this behavior. Is it because the system is volatilizing at high temperatures? What other methods could I use to sample high-energy structures? Is doing NVT simulations a reasonable choice for such cases?
I understand that what I am observing is not Lammps’ fault; however, I would highly appreciate it if anyone could provide any insight into my problem.
Here is a sample input file I am using. I would be happy to provide more information if needed.
log log.lammps units real atom_style charge atom_modify map array boundary p p p read_data input.pos pair_style reaxff NULL pair_coeff * * ffield.reax.FC H C N fix 1 all qeq/reaxff 1 0.0 10.0 1.0e-6 reaxff thermo 10 thermo_style custom step temp pe etotal fmax fnorm cella cellb cellc cellalpha cellbeta cellgamma press vol fix 3 all box/relax aniso 0.0 minimize 0.0 1.2 200000 200001 unfix 3 dump myDump all custom 50 output.dat id type element x y z fx fy fz dump_modify myDump element H C N fix 2 all npt temp 500 500 $(100*dt) aniso 1.0 1.0 $(1000*dt) timestep 0.5 run 200000