[lammps-users] about pair_lj/gromacs

I am confused by the note of pair_lj/gromacs.doc.

Note that sigma is defined in the LJ formula as the zero-crossing distance for the potential, not as the energy minimum at 2^(1/6) sigma.

MARTINI force field says the sigma_ij represents the effective minimum distance of approach between two particles. Does this mean the sigma_ij is equal the sigma of LAMMPS?

If the parameters of MARTINI force field sigma_ij = 0.47nm, The parameters have to set to be 0.47nm/ (2)^(1/6).

What is the difference of sigma definition between LAMPPs and Gromacs refferring to pair_lj/gromacs? I am confused.

I don't know about MARTINI, but the comment is there on both
the lj/gromacs and lj/charmm pages because some FFs define
sigma differently than it is defined in the formulas LAMMPS
is using. That is true for CHARMM, nor sure about GROMACS.

In CHARMM, they define "sigma" as the minimum of the potential,
so you would want to use a smaller sigma (by a factor or 2^(1/6)) as
LAMMPS input, since its formula is the zero-crossing, not the min.
The tools/ch2lmp tool does this conversion for you.

Steve