Ion non-bonded parameter mismatch between charmm36 and charmm36 for gromacs

User discussions
1

GROMACS version:Any above and including 2020.1
GROMACS modification: No

In toppar_water_ions.str from toppar_c36_jul21.tgz [http://mackerell.umaryland.edu], (R_min)/2 for POT :1.76375 Angstrom, for CLA: 2.27 Angstrom.

In charmm36-jul2021.ff.tgz, (CHARMM forcefield in gromacs format, downloaded from same website),from ffnonbonded.itp, I find the sigma for POT as 0.314264522824 nm and for CLA 0.404468018036 nm

The values in each file corresponding to each atom are simply not related by conversion between Angstrom to nm and a factor of 2 (R_min/2 to R_min which is sigma ) .

Can I get help regarding why this is happening?

Attaching the corresponding files as a data file.

ffnonbonded.dat (2.6 MB)
toppar_water_ions.dat (11.9 KB)

2

Rmin and σ are not the same, they need to be converted.

σ = 2-1/6 × Rmin

So for CHARMM, to transform Rmin/2 to σ and transform the units:

σ = 2 × 0.1 × 2-1/6 × Rmin/2

3

Oops! My bad.
Thank you!