Hi everyone,

I’m trying to perform brownian dynamics in lammps of quenched Lennard-Jones- like fluids. I read in the documentation page that this is done by using fix NVE and fix langevin together. I’d like to see gelification of this fluid.

I’m trying to reproduce the simulations performed in http://www.tandfonline.com/doi/abs/10.1080/08927029908022123

My system consisted of 864 spherical particles of diameter sigma (sigma = 1) in an implicit fluid. I chose the traditional time unit for colloidal liquids (a²/D), where D is the self diffusion coefficient. I simulated 82 a²/D , in colloidal time units. Being D = KbT/ coeff_friction, I set coeff_friction = 1, So D = T.

I equilibrated my system at T = 2.0 (homogeneous liquid) so I quenched to 0.7 and ran the simulation. I used the 12-6 Lennard-Jones potencial for pairwise interactions.

By changing the timestep I observe that the system evolves in different ways and this is not what I expect to happen.

If I chose very low timesteps the system seems to reach equilibrium in the earlier timesteps and I dont see any gelification.

My question is : How can I chose the correct timestep of this type of simulation?

I send you my code and graphics I obtained for RDF using different timesteps.

Thank you so much for your attention.

units lj

dimension 3

atom_style atomic

neigh_modify delay 0 every 1

#settings

variable N equal 864

variable T equal 0.7

variable phi equal 0.16

variable cuttof equal 2.5

variable epsilon equal 1

variable sigma equal 1

variable a equal {sigma}/2.0
variable cuttof equal 2.5*{sigma}

variable rho equal 6*{phi}/PI variable L equal ({N}/${rho})^(1.0/3.0)

variable D0 equal T
variable coeff_fric equal 1.0
variable dt_equilibration equal 8e-4*{a}*{a}/{D0}
variable damp_equi equal 100*${dt_equilibration}

# problem setup

region simbox block 0 $L 0 $L 0 $L

create_box 1 simbox

create_atoms 1 random $N 10 simbox

variable mass_par equal 1.0

mass * ${mass_par}

velocity all create 2.0 10 dist gaussian

pair_style mie/cut {cuttof} pair_coeff * * {epsilon} ${sigma} 12 6

minimize 1.0e-3 1.0e-3 1000 10000

reset_timestep 0

# equilibration

fix 1 all nvt temp 2.0 2.0 ${damp_equi}

variable equil_steps equal 400000

timestep {dt_equilibration}
thermo 1000
run {equil_steps}

reset_timestep 0

#Production

unfix 1

velocity all scale ${T}

variable damp equal {mass_par}/{coeff_fric}

variable dt equal 1.0e-5*{a}*{a}/${D0}

variable total_steps equal round(81.9*{a}*{a}/{D0}/{dt})

variable dump_interval equal round(3.2*{a}*{a}/{D0}/{dt})

fix NVE all nve

fix 1 all langevin {T} {T} ${damp} 48279 zero yes

dump 1 all custom ${dump_interval} dump_struc.lammpstrj id type x y z

thermo 1000

run ${total_steps}

RDf.pdf (53 KB)