# Questions about input file of simulation of couette flow

Dear all,

I’m working on simulation of couette flow of liquid of argon in a 2d channel and I started from example input of LAMMPS. In manual, it says" For style lj, all quantities are unitless. Without loss of generality, LAMMPS sets the fundamental quantities mass, sigma, epsilon and Blotzmann constant=1". For liquid argon, mass of one atom is 6.63*10-26 kg and relative atomic mass is 39.9 gr/mol.

My question is that what value should I consider for mass in lj units? 1 or 39.9 or 6.63*10-26 kg. I saw some other people’s input files that all of them have chosen 1 for mass, sigma and epsilon in lj units. However, if I chose 1 for all of mentioned parameters in lj units, how can I distinguish that this input file is for argon or other materials?

My second question is that how can I determine number of molecules (or atoms) in my input file? In input file, mass is only determined by the user.

I really appreciate for your help.
Regards,
Hamed.

Dear all,

I'm working on simulation of couette flow of liquid of argon in a 2d
channel and I started from example input of LAMMPS. In manual, it says" For
style lj, all quantities are unitless. Without loss of generality, LAMMPS
sets the fundamental quantities mass, sigma, epsilon and Blotzmann
constant=1". For liquid argon, mass of one atom is 6.63*10-26 kg and
relative atomic mass is 39.9 gr/mol.
My question is that what value should I consider for mass in lj units? 1
or 39.9 or 6.63*10-26 kg. I saw some other people's input files that all of
them have chosen 1 for mass, sigma and epsilon in lj units. However, if I
chose 1 for all of mentioned parameters in lj units, how can I distinguish
that this input file is for argon or other materials?

​you cannot. in reduced units, only relative differences matter. this is
the nature of ​reduced units. please note that how you equivalence energy,
mass and length to real affects derived properties like the unit of time.
so if you equivalence the length unit sigma to a different real length, or
the mass unit to a different real mass, this will result in a change of how
the time unit has to be converted to real time. this can be easily
rationalized. if you are looking at a motion from further away, everything
looks smaller (=> different length units) and moves slower (different time
unit).

if this all confuses you, just use units command and use real (or metal or
whatever) units and be done with it.

My second question is that how can I determine number of molecules (or
atoms) in my input file? In input file, mass is only determined by the user.

please explain, ​what has setting the mass to do with the number of
molecules or atoms? the number of atoms (or molecules) is determined by how
your "create" (i.e. by the lattice and the volume that you fill from that
lattice) or input them (i.e. by what is written in the data file).

​axel.​