Because size and shape can affect the chemo-physical properties of nanoparticles, we extend the use of geometrical descriptors to sequence a genome of monometallic nanoparticles. Selecting the generalised coordination number as a descriptor, the derived geometrical genome distinguishes, catalogues, and counts the variety of adsorption sites available on each isomer with a diameter up to 10 nm, therefore it depends on the nanoparticle size and shape. This procedure allows us to elucidate the effects of morphological diversity within a sample and those of thermally activated structural rearrangements among isomers on nanocatalyst activity. By screening the geometrical genome of archetypal shapes, we forecast Pt stellated twinned nanoparticles, elongated along their five-fold axis and with their shortest diameter of ∼2 nm, as optimal candidates for the electro-reduction of molecular oxygen at room temperature, in agreement with available experimental data.