Chris Neale wrote: > My apologies, The commands should have been: > > trjconv -f a.trr -o b.trr > gmxdump -f a.trr > a.dump > gmxdump -f b.trr > b.dump > diff a.dump b.dump > ab.diff Better still trjconv -f a.trr -o b.trr gmxcheck -f a.trr -f2 b.trr > ab.diff Similar goes for .tpr or .edr files. Mark