# charmm 12-6 potential

Dear lammps users,
I have came across a confusion regarding the pair_style lj/charmm variants
In the MacKerell paper (original charmm paper) the 12-6 format is eps*[(Rmin/rij)^12 - (Rmin/rij)^6] , as I think the Rmin=sig

In lammps, the potential is defined as
4eps[(sig/rij)^12 -(sig/rij)^6 ]
http://lammps.sandia.gov/doc/pair_charmm.html

this is while some other softwares yet define the potential as

eps*[(sig/rij)^12 - 2*(sig/rij)^6]

http://www.ks.uiuc.edu/Research/namd/2.9/ug/node22.html

could anyone make a comment on these
Thanks
FSE

charmm_MacKerell.pdf (308 KB)

You can convert from one form to the other. See attached picture.

Jan-Michael

You can convert from one form to the other. See attached picture.

Jan-Michael

Dear lammps users,
I have came across a confusion regarding the pair_style lj/charmm variants
In the MacKerell paper (original charmm paper) the 12-6 format is
eps*[(Rmin/rij)^12 - (Rmin/rij)^6] , as I think the Rmin=sig

this has to be a typo and Rmin != sigma

the CHARMM force field parameter files have in them:

V(Lennard-Jones) = Eps,i,j[(Rmin,i,j/ri,j)**12 - 2(Rmin,i,j/ri,j)**6]

jan-michael has already shown you how to interconvert these
based on the fact that Rmin = 2^(1/6) * sigma for a 12-6 LJ potential.

axel.