# [lammps-users] lj/cut/coul/cut pair_style

Dear Friends

If I need to use lj/cut/coul/cut pair_style, what should I set energy conservation constant ‘C’?
Dielectric constant is a unitless number. Therefore for both ‘real’ units command and ‘lj’ one, the number is equal. Do you agree?

Thanks alot for your time and attention.

Saly

If you mean the prefactor in the Coulomb eq, then you don't set
it. LAMMPS sets it internally to convert that eq into
the energy units you are using.

Steve

Dear Steve

Dear Steve

Do you mean that for running the attached paper, Only I should use

instead of just attaching a paper and let people figure out by themselves
what you are trying to do, how about giving a short summary.
begin able to describe what you want to do is the first step
of being able to do a meaningful simulations, or the other way around...

lj/cut/coul/cut pair_style with its cut offs and It should not set the
dielectric constant for Na+, Mg+,.....?

the dielectric constant to be used in MD is the one of the "background"
medium. for all atom simulations (regardless of in gas phase, liquid phase
or solid phase and what material) that medium is vacuum, so you have
to use that, which is the default in LAMMPS. the numerical value for the
constants that are needed to convert interactions between charged particles
into energies and forces are properly preset, too, due to the choice of
the units flag.

if you have reasons to believe that things should be different
please explain them in more detail.

axel.

Dear Friends

For example the dielectric constant of Na+ is 6.1. To define the potential for Na+ and Na+, should I write in my input file the following:

pair_coeff 1 1 lj/cut/coul/long 0.099 2.586 6.1

or

pair_coeff 1 1 lj/cut/coul/long 0.099 2.586

Regards

Saly

Dear Friends

For example the dielectric constant of Na+ is 6.1. To define the potential
for Na+ and Na+, should I write in my input file the following:

please _first_ show why that would be necessary.
i already clearly explained to you that you _must_
use the vacuum dielectric constant for all-atom simulations.

if you keep ignoring advice, you'll be soon running
out of people that will (try to) answer.

pair_coeff 1 1 lj/cut/coul/long 0.099 2.586 6.1

this is _complete_ nonsense. the third number is the pairwise
lennard-jones cutoff radius and the documentation clearly
states that. you cannot just put numbers in random places
and hope that the code will do something useful with it.

or

pair_coeff 1 1 lj/cut/coul/long 0.099 2.586