# FENE potential K parameter

Hello,

I have a question about FENE potential.Based on the lammps manual :

``````bond_style fene
bond_coeff 1 30.0 1.5 1.0 1.0
``````

K parameter (I suppose it’s bond stiffness) has been set to 30.

what is the unit systems in this example?Is K equal to 30 in real unit or LJ unit?

thanks

Hello,

I have a question about FENE potential.Based on the lammps manual :

bond_style fene
bond_coeff 1 30.0 1.5 1.0 1.0

K parameter (I suppose it's bond stiffness) has been set to 30.

what is the unit systems in this example?Is K equal to 30 in real unit or
LJ unit?

could be either or something else. please consult the documentation.
starting here: LAMMPS Molecular Dynamics Simulator

In other words, you choose the units in your input script.

Then the K param (or param for any command), is

in those units.

Steve

yes, I know that.I just was wondering if there is any option which I can redefine a parameter unit in a unit system.For example, I redefine the unit of length in LJ system from 6 nm to 1nm

thanks

Not sure what you are asking. For any other LAMMPS units
than LJ, distance units have a fixed meaning, e.g. Angstroms
in “real” units. If you are asking can you make a setting
in your input script and LAMMPS will now consider
the length unit to be 6 Angstroms, e.g. when reading
atom coords from a data file, then the answer is no.

Note that for input script commands that require a distance
(in Angstroms), you can specify the param with a variable
that does something like

variable scale equal 6
variable dist equal 2*{scale} command_foo arg1 arg2 {dist}

So that to modify the behavior of many commands
you just have to change the scale=6 to something else.
You could set scale from the command-line when

For LJ units, distance can be a function of the density
of your system. So the LJ “sigma” can have different
effective values, e.g. depending on the lattice command
you define. Many LAMMPS input command use “lattice units”
reduced LJ units mean in MD simulations.

Steve

Now I’m getting a little bit concerned about lj unit. Suppose that I don’t use a lattice command. The standard units are the leonard Jones unit system anyway, right? Like unit of length in lj is equivalent to 6 nm, Unit of energy parameter is equivalent to 1.37. 10 ^ -20 j and…
This is what I expect from lj unit system
Best,
Ata.

Now I'm getting a little bit concerned about lj unit. Suppose that I don't
use a lattice command. The standard units are the leonard Jones unit system
anyway, right?

the lattice command and the units command are two completely different
entities. nothing in the units changes when you change the lattice.
units can only be set once

Like unit of length in lj is equivalent to 6 nm, Unit of
energy parameter is equivalent to 1.37. 10 ^ -20 j and....
This is what I expect from lj unit system

that is an unusual expectation. where is this documented? the LAMMPS
documentation is very clear about what it understands under reduced
units. LAMMPS Molecular Dynamics Simulator

thus for "units lj" (aka reduced units), the unit of length is 1
sigma, the unit of energy is 1 epsilon. there is *no* direct relation
to absolute units, that is the whole point of using reduced units. so
a length of 1.0 in reduced units can represent *any* length, be it 1
angstrom, 6 nanometers or 5 miles only that a sigma of 1.0 in the pair
coeff command represents the same distance as a length of 1.0 in the
bond_coeff command.

axel.