gmx mindist computes the distance between one group and a number of other groups. Both the minimum distance (between any pair of atoms from the respective groups) and the number of contacts within a given distance are written to two separate output files. With the -group option a contact of an atom in another group with multiple atoms in the first group is counted as one contact instead of as multiple contacts. With -or, minimum distances to each residue in the first group are determined and plotted as a function of residue number.
With option -pi the minimum distance of a group to its periodic image is plotted. This is useful for checking if a protein has seen its periodic image during a simulation. Only one shift in each direction is considered, giving a total of 26 shifts. It also plots the maximum distance within the group and the lengths of the three box vectors.