[lammps-users] water simulation

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1

Hello
Dear Ahmed,

I intend to simulate pure water. in order to do this, I have written a script as below:

units real
neigh_modify delay 2 every 1
atom_style full
bond_style harmonic
angle_style charmm
pair_style lj/charmm/coul/long 8 10
kspace_style pppm 1e-4
read_data solvate.data
pair_coeff 1 1 0.0460 0.4000 .0460 0.4000 # H
pair_coeff 2 2 0.1521 3.1507 0.1521 3.1507 # O
pair_coeff 1 2 0.0836 1.7753 0.0836 1.7753 # H-O
special_bonds charmm
minimize 1e-9 1e-11 100000 1000000
fix 1 all nve
fix 2 all shake 1e-6 500 0 m 1.0 a 1
fix 3 all langevin 300.0 300.0 800 123456
velocity all create 300.0 12345678 dist uniform
thermo 10
thermo_style multi
timestep 0.5
run 400000
unfix 1
unfix 3
fix 4 all npt 300.0 300.0 2 xyz 1.0 1.0 60 drag .05
run 4000000

I have some problem on set parameters : Tdamp , Pdamp and drag factor. have there any rule for set this parameter?
I try to set this parameter till temp =300 and press=1 in real unit. temp can be balanced but press no. would you help me.

Thanks a lot
Yasti

2

The Tdamp and pdamp factor give times for the “relaxation” of the system, in terms of the units selected. So, in real units, Tdamp = 0.2 fs, which is an incredibly fast time. This means that you’re going to have rather violent temperature corrections. Even 60 fs for the pdamp constant is on the fast side. Normally, I’ve seen values of 100-500 fs for Tdamp, and 500-1000 fs for pdamp. Also, drag is typically between 0.2 and 2.0.

–AEI