  Characterizing the melting behaviour of monometallic nanoparticles is a great
challenge from both the experimental and the theoretical point of view. To this
end, we disclose a universal signature based on the cluster's pair distribution
function, a measurable quantity from X-ray experimental analysis tools. From a
systematic investigation of metallic nanoparticles of different chemical
species (Ni, Cu, Pd, Ag, Au and Pt), in a wide size range (146 to 976 atoms),
and using both crystalline and five-fold twinned shapes as initial
configurations, it emerges that the melting transition is signalled by the
disappearance of a peak at the second nearest neighbours in the pair
distribution function. To this end, we show that the relative cross-entropy of
the pair distribution function between a "cold" and a "hot" reference structure
correlates with their caloric curves, thus also presenting a quasi-first order
transition at the melting temperature. Finally, we demonstrate the fruitful
application of the proposed structural characterization method to identify the
melting temperature of nanoparticles in a strongly interacting environment,
where low-symmetry solid and melted phases are quasi-degenerate in energy.
