NPT Equilibration of Solution

LAMMPS LAMMPS Beginners
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1

Hello,
I want to equilibrate a solution of methanol and fluorobenzene using NPT equilibration and calculate the radial distribution function (RDF) after that. The temperature is fixed at all the time step but the pressure is not. Is there any other method to equilibrate the solution system apart from this (pressure and temperature have to be constant during the calculation of RDF)? My input script is:
##################################################
units real
boundary p p p

atom_style full
bond_style harmonic
angle_style harmonic
dihedral_style opls
special_bonds lj/coul 0.0 0.0 0.5
pair_style hybrid lj/cut/coul/long 12.0 12.0
pair_modify mix geometric tail yes
kspace_style pppm 1.0e-5
read_data data.lmp

remove pair style if not using hybrid

pair_coeff 1 1 lj/cut/coul/long 0.070000 3.550000 # C00 C00
pair_coeff 2 2 lj/cut/coul/long 0.070000 3.550000 # C01 C01
pair_coeff 3 3 lj/cut/coul/long 0.070000 3.550000 # C02 C02
pair_coeff 4 4 lj/cut/coul/long 0.030000 2.420000 # H03 H03
pair_coeff 5 5 lj/cut/coul/long 0.070000 3.550000 # C04 C04
pair_coeff 6 6 lj/cut/coul/long 0.030000 2.420000 # H05 H05
pair_coeff 7 7 lj/cut/coul/long 0.070000 3.550000 # C06 C06
pair_coeff 8 8 lj/cut/coul/long 0.030000 2.420000 # H07 H07
pair_coeff 9 9 lj/cut/coul/long 0.070000 3.550000 # C08 C08
pair_coeff 10 10 lj/cut/coul/long 0.030000 2.420000 # H09 H09
pair_coeff 11 11 lj/cut/coul/long 0.030000 2.420000 # H0A H0A
pair_coeff 12 12 lj/cut/coul/long 0.060000 2.900000 # F0B F0B
pair_coeff 13 13 lj/cut/coul/long 0.030000 2.500000 # H000 H000
pair_coeff 14 14 lj/cut/coul/long 0.065999 3.500000 # C001 C001
pair_coeff 15 15 lj/cut/coul/long 0.030000 2.500000 # H002 H002
pair_coeff 16 16 lj/cut/coul/long 0.030000 2.500000 # H003 H003
pair_coeff 17 17 lj/cut/coul/long 0.170000 3.120000 # O004 O004
pair_coeff 18 18 lj/cut/coul/long 0.000000 0.000000 # H005 H005

fix 1 all npt temp 298.0 298.0 100.0 iso 1.0 1.0 1000.0
neighbor 2.0 bin
timestep 0.5
variable TK equal 298.0
variable PBAR equal 1.0
velocity all create ${TK} 12345
thermo 1000
thermo_style custom step cpu etotal ke pe evdwl ecoul elong temp press vol density
dump 3 all atom 500 dump.flow
run 1000000

2

This has been discussed many times. Both, temperature and pressure fluctuate, but pressure fluctuates more because condensed systems are not very compressible. Fluctuations go down with increased system size. Please see previous discussions of the topic for detailed explanations and suggestions.

3

Thank you for your reply. I have gone through the discussions but the pressure fluctuation is around -300 to 300 in my case. Is there any other problem with my script? Is there any other way to reduce this fluctuation?
Thanks in advance.

4

300 what?

How large is your system?

5

1000 fluorobenzene and 9000 methanol molecules in a 90x90x90 box.

6

That level of pressure fluctuations for that kind of system size doesn’t seem to be outlandish. When running the scaled “rhodo” benchmark example with -v z 2 the system has 64000 atoms and the pressure oscillates between -600 atm and 600 atm see attached graph.

in.rhodo.scaled.Press

7

Thanks a lot.