[gmx-users] Conversion between nonbonded charmm NBFIX and gromacs cross terms

[Marlon Sidore]

Hello,

I am currently translating non-bonded parameters from CHARMM to GROMACS,
which include/necessitate cross terms for the forcefield I'm interested in
(MARTINI).

I am struggling at the conversion from the NBFIX lines, which have this
shape:
"P5 P5 -1.33843000 5.27557163
P5 Nda -1.19503000 5.27557163"
To something gromacs like, which have this shape:
"  P5    P5  1   0.24145E-00     0.26027E-02 ; supra attractive
  P5    Nda     1   0.21558E-00     0.23238E-02 ; attractive"

I think I understood that CHARMM uses Rmin and Emin(epsilon) and GROMACS
uses c6 and c12. But I'm currently stuck at how to translate the former
into the latter, as it doesn't seem like a simple scaling - since in this
example 2 numbers from CHARMM are the same while none are the same in
GROMACS.

Thanks for your help

Marlon Sidore


PhD Student
Laboratoire d'Ingénierie des Systèmes Macromoléculaire (LISM)
CNRS - UMR7255
31, Chemin Joseph Aiguier
13402 cedex 20 Marseille
France