Hey Nahren, > > sed -i '/^ATOM.*O2/d' DIMER.pdb > > Am i going wrong here....? You should try :) But it looks quite okay to me ;) Cheers, Tsjerk -- Tsjerk A. Wassenaar, Ph.D. post-doctoral researcher Molecular Dynamics Group Groningen Institute for Biomolecular Research and Biotechnology / University of Groningen The Netherlands