  We present a systematic analysis of molecular oxygen (O$_2$) adsorption
trends on bimetallic PtNi clusters and their monometallic counterparts
supported on MgO(100), by means of periodic DFT calculations for sizes between
25 up to 58 atoms. O$_2$ adsorption was studied on a variety of inequivalent
sites for different structural motifs, such as truncated octahedral (TO),
cuboctahedral (CO), icosahedral (Ih) and decahedral (Dh) geometries. We found
that O$_2$ prefers to bind on top of two metal atoms, parallel to the cluster,
with an average chemisorption energy of 1.09 eV (PtNi), 1.07 eV (Pt) and 2.09
eV (Ni), respectively. The largest adsorption energy values are found to be
along the edges between two neighbouring (111)/(111) and (111)/(100) facets;
while FCC and HCP sites located on the (111) facets may show a chemisorption
value lower 0.3 eV where often fast O$_2$ dissociation easily occurs. Our
results show that, even though it is difficult to disentangle the geometrical
and electronic effects on the oxygen molecule adsorption, there is a strong
correlation between the calculated general coordination number (GCN) and the
chemisorp- tion map. Finally, the inclusion of dispersion corrections (DFT-D)
leads to an overall increase on the calculated adsorption energy values but
with a negligible alteration on the general O$_2$ adsorption trends.
