[lammps-users] lattice command with LJ units

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1

Dear LAMMPS users,

I want to use

lattice sq2

like in the NEMD example input file. However I don't understand the lattice documentation for LJ units:

** For unit style /lj/, the scale argument is the Lennard-Jones reduced density, typically written as rho*. **

Q: In this sq2 case is rho* the number density (per unit area) of the two basis atoms?

Q: What is the difference between rho and rho*?

** Effectively, this means that if LJ particles of size sigma = 1.0 are used in the simulation, the lattice of particles will be at the desired reduced density. **

Q: Does the size of particles mean it's diameter? How could the diameter of the particle be sigma if the lattice constant is sigma as well?

Q: Does the lattice command set the diameter of particles?

Q: Does the behaviour of the lattice command change , when I define the create_box region in box units rather than lattice units?

With kind regards,
Thomas Wagner.

2

You probably need to read a textbook discussion of reduced units,
e.g. in Allen and Tildesly. The simplest view of this for LJ units is
that the particles always have diameter sigma = 1.0. When you specify
rho* as 0.6 (for example), then LAMMPS will choose a lattice
spacing (bigger than 1.0) so that the number density is 0.6, according
to rho* = N/V sigma^3, but just assume sigma = 1. The lattice
constant is the cube root of V. N is the number of atoms in the cubic
unit cell. Cubes become squares in 2d.

Steve