Respectable Mark Thank you very much for your immediate response. I have added new atom type in the ffgmx.atp file and also modified ffgmx.itp, ffgmxnb.itp. After that the pdb2gmx worked. I used genbox to add water in the oxygen molecule system. For now I have taken only 2 oxygen molecules with the box size 1 nm. I have run the energy minimization. The potential energy came to be -1.632 KJ/mole. Can I be hopeful that I will be able to go to production job, and Most important thing is I need value of O-O bond constant for harmonic potential. Could you provide me with that. Sunil
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