gmx mdmat makes distance matrices consisting of the smallest distance between residue pairs. With -frames, these distance matrices can be stored in order to see differences in tertiary structure as a function of time. If you choose your options unwisely, this may generate a large output file. By default, only an averaged matrix over the whole trajectory is output. Also a count of the number of different atomic contacts between residues over the whole trajectory can be made. The output can be processed with gmx xpm2ps to make a PostScript (tm) plot.