[gmx-users] CHARMM -> gromacs epsilon and sigma conversiion factors

[ABEL Stephane 175950]

Hi all,
 
I would like to add new  atom types in the charmm27.ff for futures simulations of glyolipids in water with gmx 4.5.1. I have finished to convert the bonded in GROMACS format. However, I am little puzzled with the vdW paramaters. I would like to know how to convert them (i.e. what are the conversion factors used to convert the CHARMM sigma and epsilon parameters to gromacs sigma and epsilon values ?).
 
For example for the chloride atom CLA
 
CHARMM 27 
 
; atom  ignored    epsilon      Rmin/2   ignored   eps,1-4       Rmin/2,1-4
CLA      0.0       -0.150     2.27      ! chloride
 
GROMACS ffcharmmnb.itp
 
;name at.num mass charge ptype sigma epsilon
CLA 17 35.450000 -1.00 A 0.404468018036 0.6276 
 
Thanks in advance for your help
 
Stefane