[lammps-users] About LJ units

Dear all,

I am confused by the LJ units in LAMMPS, e.g. what is the specific material simulated in the example of the crack? And what are the realistic meaning for these velocity, temperature, mass, etc. used in LJ units?

Will anyone recommand me a tutorial about how to set simulation parameters for LJ units?

Best regards,
Damien
2008-07-23

Dear all,

I am confused by the LJ units in LAMMPS, e.g. what is the specific
material simulated in the example of the crack? And what are the
realistic meaning for these velocity, temperature, mass, etc. used in
LJ units?

have a look at the documentation of the units command.
you can re-generate "realistic" values by multiplying
with real epsilon, sigma, mass and k-boltzman in the
appropriate ways.

Will anyone recommand me a tutorial about how to set simulation
parameters for LJ units?

all you have to do is sit down and think a little about it.

when using LJ units you simulate _all_ systems with the
same ratio of mass, sigma, epsilon and kB.

basically instead of 1 angstrom you use the sigma value
of one reference particle as fundamental length. the sigma
for the second is then the ratio. same for mass, epsilon
and so on. the documentation also lists how to compute
derived physical quantities from that.

it is basically the same as atomic units in quantum chemistry... :slight_smile:

cheers,
   axel.

Will anyone recommand me a tutorial about how to set simulation parameters for LJ units?

See Allen & Tildesly, appendix B.

Steve