I find that fix_npt in LAMMPS does not work well when simulating a nanocrystalline metal.
NPT with Parinello-Rahman box control should be released
by the end of the summer.
But the NPT fix should be satisfied of simulating a box using a unixial constant stress (pressure) control while not change the shape of box. In terms of nc, I have some publications, e.g. 'Cao AJ, Wei YG, “Atomistic simulations of crack ucleation and intergranular fracture in bulk nanocrystalline nickel”, PHYSICAL REVIEW B, 76,024113 '.
BTW, I used the early version “PARADYN” not LAMPPS. I’m currently learning Lampps, but I guess they are qrite similar in algorithm.
Kaiguo, let me know your specific problem of simulations.