measure MSD of two kind atom together

dear lammps users
I want measure MSD of two kind atom together

i use this command ;

compute MSD H & O msd com yes

that H and O are hydrogen and oxygen

i see this error :
Unknown compute style

any one have idea how can I measure togheter?

best

Where did you get the idea that this command would work? The documentation for group explains how you should group atoms together. You can then use that group in compute MSD.