Dear Friends,

I am trying to simulate a metallocene ligand with a metal ion between two Cp rings (Refer image). For ff for metal ion and ligand I have generated using QM calc.

Now I have searched the literature for ff parameters if available, but unfortunately i didn't found any in gromacs. But for CHARMM there are few papers which have successfully performed MD for ferrocene. They have shown four different possibilities for doing the same (image attached with description), where by the first three involves the usage of dummy atom to keep the metal bonded in between the Cp ring. The fourth one seems to be rational and I want to use this to avoid the dummy atom usage.

The paper have mentioned the procedure " Therefore, in the present model the substituted Cp rings and the iron atom interact with each other with both nonbonded terms and also distance restraints that hold together the Cp rings and the iron atom. The CHARMM potential energy has a restraint term that prevents large motions of individual atoms. In the present case, all C atoms on the Cp rings were constrained to be equidistant from the central iron (2.05 A) with a harmonic constraint force constant of 100 kcal mol
-1 A -
2 ."

Can anyone help me how to apply these nonbonded constraints in gromacs? Any help is highly appreciated.