Measuring the similarity of spectra of molecular graphs

1-4Three measures of the similarity of the spectra of molecular graphs are considered. Two of them are previously known: the "almost-isospectrality measure", aim, and the "angle of isomerization", ais. We now introduce a third such quantity-the "vectorial isospectrality measure", vim. The previous definitions of aim and ais are modified so as to make them applicable to arbitrary pairs of molecular graphs with equal number of vertices. We establish relations between aim, ais & vim, and point out some of their other properties