Hey Juhi! If your molecule does not exist it doesn't mean that parameters do not exist. In general you can draw your molecule, then assign to every atom a type according to Gromos FF "philosophy", then write an rtp-residue for this molecule. Such an rtp-file you may find in the respective folder for a force field. Normally it is in gromacs-version/share/top. Then you can try to generate a new itp-file using pdb2gmx routine.
I want to simulate DNA covalently bonded to a ligand. I read GROMACS Tutorial (Protein-Ligand Complex) from Justin A. Lemkul, Ph.D. in this tutorial the ligand and receptor is not covalently bonded.
do you know whether I should parameterize the ligand and DNA separately and then merge them together? In this case, might I lose a covalent bond between the DNA and the ligand? Could you please kindly guide me on how I can simulate the DNA covalently bonded to a ligand?