[lammps-users] lj/charmm/coul/charmm problems

Dear all,

I am having problems getting pair_style lj/charmm/coul/charmm to work. TIP3P water is behaving like a solid (molecules only move around there initial positions and the water roughly maintains the shape of the simulation box) instead of a liquid at 300 K. I have tried simulations with various sets of cutoffs, NPT, NVT, NVE, with and without SHAKE, and at different temperatures and I get similar behavior in every case. At 1500 K, where boiling should occur, the system volume actually decreases when using NPT. When I use pair_style lj/charmm/coul/long/opt, I have no problem. The script I have been using is below. Unless there is something wrong in my script that I did not notice, there is likely a bug. I would not expect lj/charmm/coul/charmm to behave radically different from lj/charmm/coul/long/opt. I am using the 7Sept09 version.

Thanks for any help,
Brian Novak

Created by charmm2lammps v1.8.1 on Thu Sep 10 16:16:54 CDT 2009

units real
neigh_modify delay 0 every 1 check yes

atom_style full
bond_style harmonic
angle_style charmm

#pair_style lj/charmm/coul/long/opt 10 12
#pair_modify mix arithmetic table 12
#kspace_style pppm 1e-5

pair_style lj/charmm/coul/charmm 10 12
pair_modify mix arithmetic

#read_data solvate.data
#read_restart eminpppm.restart
read_restart emin.restart

special_bonds charmm
fix 2 all shake 1e-6 500 0 m 1.0 a 1
fix 1 all npt 300.0 300.0 100.0 xyz 1.0 1.0 1000.0
#fix 1 all nvt 300.0 300.0 100.0
#fix 1 all nve
velocity all create 300.0 12345678 dist gaussian

thermo 100
thermo_style custom step temp press vol pe ke etotal evdwl ecoul ebond eangle
timestep 2.0

restart 1000 solvate.restart1 solvate.restart2
dump 1 all dcd 5000 solvate.dcd
dump_modify 1 unwrap yes

run 200000
#minimize 1.0e-4 1.0e-6 1000 100000

lj/charmm/coul/charmm is a cutoff Coulomb interaction
lj/charmm/coul/long/opt with pppm is an infinite-range Coulomb interaction

So they could give different behavior. As to why the short-range
is freezing your water, I don't know. Are you sure you've parameterized
TIP3P for the short-range (cutoff case) and not long-range water?