I'm using g_traj in gromacs to convert my .xtc file to .xvg file. These are my coordinates for my water molecules. The problem is, I want to do this for oxygen atoms and for hydrogen atoms separately, but when I run the g_traj command, my only options are system and SOL. I need a way for it to recognize the fact that I have 2 separate atom types, the coordinates of which I want in separate files. Any suggestions?