yes, when creating the topology file i added atom O2 to the .rtp file when it said atom O2 is missing in atomstype regards, ram -- View this message in context: http://gromacs.5086.n6.nabble.com/Fatal-error-Atomtype-O2-not-found-tp4998254p4998256.html Sent from the GROMACS Users Forum mailing list archive at Nabble.com.