# Integrated LJ interaction between Nano-particles

Hello everyone,

I am trying to find out the agglomeration of nano-particles through
simulation in lammps. I have done simulation, but recently I get confused
with few things, any kind of help will be appreciated:

1. I am using integrated LJ potential through pair_style colloid.
2. I am using units nano with particle size of 50 nm and mass 255.52 ag

My questions are:-

1. I want to set sigma as the diameter of the particle, so in pair_coeff I
define like this -

pair_coeff 1 1 500.0 50.0 25.0 25.0 200.0

where, 500.0 is hamaker constant (nano unit) 50.0 is sigma( which I set as
dia of particle) and 25.0 (radius of interacting particles). *Is this the
correct way to define pair_coefficient ?*

2. If Yes ! *Then why, while writing diameter of the particles* in dump
custom command I am getting it as 1.0 and radius as 0.5 ? or is it
giving in some scaled unit?

3. I want to calculate no. of particles touching each other by compute
contact/atom command and using fix ave/atom to output data.But by this
command I am getting error response of Illegal fix ave/atom command

Ill be thankfull to any kind of help for this.

Thanks and Regards

Alok
PhD Scholar
IIT Bombay

Hello everyone,

I am trying to find out the agglomeration of nano-particles through simulation in lammps. I have done simulation, but recently I get confused with few things, any kind of help will be appreciated:

1. I am using integrated LJ potential through pair_style colloid.
2. I am using units nano with particle size of 50 nm and mass 255.52 ag

My questions are:-

1. I want to set sigma as the diameter of the particle, so in pair_coeff I define like this -

pair_coeff 1 1 500.0 50.0 25.0 25.0 200.0

where, 500.0 is hamaker constant (nano unit) 50.0 is sigma( which I set as dia of particle) and 25.0 (radius of interacting particles). Is this the correct way to define pair_coefficient ?

the meaning of the arguments to pair_coeff are explained in detail in then pair style colloid documentation.

1. If Yes ! Then why, while writing diameter of the particles in dump custom command I am getting it as 1.0 and radius as 0.5 ? or is it giving in some scaled unit?

the per-atom properties of radius or diameter only apply to finite size particles (e.g. with atom style sphere). the colloid pair style however, does not use this information and works with atom style atomic. the radius of the large particles is connected to the atom type and specified in the pair_coeff parameters. thus outputting radius or diameter is meaningless. again, please read the pair style colloid documentation carefully, which explains this.

1. I want to calculate no. of particles touching each other by compute contact/atom command and using fix ave/atom to output data.But by this command I am getting error response of Illegal fix ave/atom command

impossible to say. please note, that the contact/atom compute does require atom style sphere and will use the radius information there (which is - as established before - is not connected to the radius specified in the pair_coeff statements). so to make this work, you must used atom style sphere (which is a superset of atom style atomic) and assign radius values to the individual atoms that are consistent with the pair_coeff settings from pair style colloid.

axel.

Thanks alot Axel !

In my case even in all cases when using Colloid potential (Integrated LJ
potential between colloid-colloid) atom style used is *Sphere. *Whether I
am missing something else or my understanding of atom_style sphere is wrong
?

In pair_style colloid I am just using Colloid- Colloid means the
interaction between particles of finite size. Which is just the integrated
LJ potential between two spherical particles.

as i mentioned, pair style colloid does not care whether you use atom style sphere and not use the radius information, but the diameter of your “large” particle is defined through the potential parameters provided in the pair_coeff command(s).

for the rest, if LAMMPS stops with an illegal xxx command error, that means, that your input is not correct. to correct, you have to compare to the manual. please make certain, that you compare to a version of the manual that matches your LAMMPS version. syntax sometimes changes as features are added to modified.

axel.

Whether or not atom_style sphere makes sense for you is for you to decide. All it does is give a finite size to your atoms that can be used to create an angular momentum for them, and some other little things. If you do not care about the finite size of your particles beyond their diameter, you probably can live without.

Thanks Stefan

Diameter and mass are two things which i need for my problem. I dont want
angular momentum as I am using integrated LJ potential which is a central
potential.
I will read in detail about atom_style sphere, Maybe I am missing something
here,

Thanks Alex also, I agree colloid potential is defined in terms of solvent
particle (point mass), this concept I was confused about. I will look if
the same potential will work if we have only colloid (no solvent), Till now
its working but its correct or not is the point.