force field

Dear All users

How Can i use a force field that Lammps isn’t included it?

Hello again. (I forgot to CC to the list.)

If you have the time to write the code yourself, it is easy to add new
features to LAMMPS. Check out:

http://lammps.sandia.gov/doc/Section_modify.html

Andrew

This might be pretty obvious, but if it is pair potential you can
tabulate the interactions and use the pair_style table. You can also
combine existing pair styles into a new interaction using pair_style
hybrid/overlay.

This might be pretty obvious, but if it is pair potential you can
tabulate the interactions and use the pair_style table. You can also
combine existing pair styles into a new interaction using pair_style
hybrid/overlay.

Good point there.

Since you have a reasonable force-field to simulate the calcite and
the alkane chains independently, you just need to decide how they
interact with each other.

If there is no reaction occurring between the Alkane chains and the
calcite, people sometimes model the interaction using ordinary
Lennard-Jones forces with the default simple ("arithmetic"
Lorentz-Berthelot) mixing rules described here:

http://lammps.sandia.gov/doc/pair_modify.html

For this, you just need to to look up reasonable epsilon and sigma
Lennard-Jones parameters for the carbon, hydrogen, calcium, and
oxygen, atoms in your system. Then you would use calculate the new
epsilon and sigma parameters commands for the 6 interactions (2x3)
between the 2 atoms in the Alkane chains and the 3 atoms in the
calcite. Then use "pair_coeff" commands to manually set the the 4
interactions (2x2) between these 4 atom types The AMBER GAFF
parameters may be a reasonable starting place to start looking.
(Warning: These mixing rules do not always work.) Search "AMBER GAFF"
on google.

Someone with chemistry knowledge may post a reply to correct this suggestion.

Got to run.
Cheers

Andrew