LAMMPS fix gcmc+fix nvt

LAMMPS LAMMPS General Discussion
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1

Hello, I use fix gcmc+fix nvt in lammps. When I tried to run it in a box with a temperature of 298.15K and a pressure of 8bar, I found that the density of CH4 inserted using GCMC did not match the data on NIST. ,why is that? I don’t know which step I wrote wrong. Can anyone help me answer it?

in.ch42 (1.8 KB)

2

Here is my input

echo screen
variable P equal 8
variable T equal 298 # initiale temperature,K
variable dt equal 0.1 # timesteps, fs

============= simulations initial parameters ================

units real
atom_style full
pair_style lj/cut/coul/long 12
bond_style harmonic
angle_style harmonic
kspace_style pppm 1.0e-4
neighbor 2 bin
pair_modify mix arithmetic tail yes
neigh_modify every 1 delay 0 check yes
region box block 0 100 0 100 0 100 units box
create_box 2 box bond/types 1 angle/types 1 extra/bond/per/atom 10 extra/angle/per/atom 10 extra/special/per/atom 10

for CH4

pair_coeff 1 1 0.0380000011 2.4499714540 # h
pair_coeff 2 2 0.1599999990 3.4745050026 # c
bond_coeff 1 340.6175 1.1050
angle_coeff 1 39.5000 106.400
mass 1 1.007970 #h
mass 2 12.011 #c
region c4_block block 0 100 0 100 0 100 units box
molecule CH4 CH4.txt
create_atoms 0 random 198 12345 c4_block mol CH4 12345
group CH4 type 1 2
timestep {dt} velocity CH4 create {T} 12345 loop local

============= initialise velocities, perform nvt on C4 ================

minimize 1.0e-5 1.0e-7 10000 100000
reset_timestep 0
compute CH4_temp CH4 temp
fix CH4_NVT CH4 nvt temp {T} {T} (100.0*dt) fix mygcmc CH4 gcmc 100 100 0 0 123456 {T} -10000 1 mol CH4 pressure ${P} tfac_insert 2 group CH4 region c4_block
compute_modify CH4_temp dynamic/dof yes
compute_modify thermo_temp dynamic/dof yes
fix_modify CH4_NVT temp CH4_temp
thermo 1000
thermo_style custom step etotal temp pe ke density atoms press
dump 1 all custom 1000 dump.GCMC.lammpstrj id mol element type q x y z ix iy iz
dump_modify 1 element H C
run 500000
unfix CH4_NVT
unfix mygcmc
write_data gcmc_restart.data

3

You may need to calibrate with fugacity coefficient because CH4 is not an ideal gas. Sources of fugacity coefficient can refer to SRK-EOS, PR-EOS, or RK-EOS. In addition, I would like to ask you a question, how is the value of tfac_insert 2 determined in your example?