Hi,
I'm a new LAMMPS user. My group at Wayne State University in Detroit, Mich. researches phospholipid bilayers, among other things and has largely used NAMD for past modeling.
I've been trying to get LAMMPS running with some basic simulations to prove out that its working as intended, before moving on to more complex simulations (phospholipids).
I have a working simulation in NAMD that simulates 300 ethane molecules at the saturation pressure for 220 K (appr. 4.9 atm). The goal of the simulation is to get an equilibriated density, and make sure it matches then NIST experimental data. This simulation involves three configuration files -- a minimization one, a heating one, and finally an equilibriation one. Having achieved good results in NAMD -- a equilibrium density of 16.29 mol/l (experimentally predicted is 16.5), I'm now trying to translate this into LAMMPS.
I ran the charmm2lammps perl script to extract my .data file from my PDB/PSF model, and then modified the input file (see attached for these files). I had to add a minimization step (expected) or otherwise the simulation would fail as atoms would go out of range, likely due to explosion. That brings me to my first question -- what kind of minimization values are generally a good idea to use (my system has 300 molecules, 2400 atoms total)??
Using a minimization, I next tried to take the simulation through a heating process at constant pressure, using the nph and temp/rescale fixes. This did not work, the simulation exploded.
I then tried directly jumping into my desired simulation temperature 220 K via the velocity command (previously I had been calling this command with a temp of 0 K, I now am using 230 K). I then run a million step simulation with a npt fix with the saturation pressure and temp of 220 K. My dampening constants for temp and pressure, respectively, are 100 and 1000 fs.
This simulation runs, but it does not go to the liquid phase, even after a long minimization. I packed the system to approximately the appropriate volume as well -- 27,000 Angstrom^3. I've tried turning up the pressure and running at above the critical temperature, but no matter what the system seems to explode as the Potential energy and volume soar up.
Looking at my files, does anyone have an idea how I can achieve my object -- to simulate this box of ethane in the liquid phase @ 220 K? What am I doing wrong?
Also, if anyone knows of good LAMMPS tutorials for beginners that they can point me to, that would be great. The limited examples that came with the distribution of LAMMPS weren't overly helpful. I believe I understand most of the commands, but given my previous difficulties, putting them together into a working simulation still seems tedious.
I appreciate any help, thanks!!
Cheers,
Jason Mick
Graduate/Ph.D Student
Wayne State University
E: [email protected]...
C: 248.978.5941
ethane_30A3_3.in (757 Bytes)
ethane_30A3_3.pdb (185 KB)