I would like to get some advice on the subtleties regarding anisotropic vs. isotropic pressure control when simulating crystals in lammps. I am attempting to measure the enthalpiy of an 8x4*8 alpha glycine crystal (1024 molecules). Using the geometry obtained from the CSD, and after applying the appropriate rotations to cartesian frame, I found that the experimental cell lengths are lx = 40.8160 ang, ly = 47.8836 ang, 40.3593 ang, with a tilt factor of fz = -16.1431, with all other tilts zero. These were the initial lengths I used in my input data file for lammps.
I initially ran an equilibration for 2ns in the npt ensemble, with the following fix applied:
fix 2 all npt temp 330.0 330.0 2000.0 aniso 1.0 1.0 1000.0
I noticed that half way through the run, the total energy dropped suddenly. I checked the geometry of the lattice, and have plotted the box lengths (lx,ly,lz) below in aniso.pdf. As you can see, the box lengths experience sudden changes, after having reached a plateau.
I suspected that this was connected to the pressure control, and ran the same simulation, but now with isotropic pressure coupling. The box lengths are plotted in iso.pdf. As you can, there is so abrupt structural change when maintaining the pressure this way.
At this point, I cannot tell which type of pressure control is more appropriate. The abrupt change with anisotropic pressure coupling to me is unsettling. However, it could just be that isotropic coupling is preventing a structural relaxation. Furthermore, the isotropic pressure coupling reproduced the equilibrium box lengths better than anisotropic coupling, which shows a flip in the ranking of the lx and lz box lengths. The force field I am using is GAFF + CNDO charges (D.W. Cheong, Y.D. Boon, Comparative Study of Force Fields for Molecular Dynamics Simulations of Alpha-Glycine Crystal Growth from Solution). In the cited work, its not clear what coupling the authors used with performing bulk crystal simulations.
Any insight regarding the selection of the coupling method, or comments about the physics I am observing, would be greatly appreciated.
aniso.pdf (21.8 KB)
iso.pdf (20.9 KB)