  Molecular dynamics simulation is employed to understand the thermodynamic
behavior of cuboctahedron (cub) and icosahedron (ico) nanoparticles with 2-20
number of full shells. The original embedded atom method (EAM) was compared to
the more recent highly optimized version as inter-atomic potential. The thermal
stability of clusters were probed using potential energy and specific heat
capacity as well as structure analysis by radial distribution function, G(r),
and common neighbor analysis (CNA), simultaneously, to make a comprehensive
picture of the solid state and melting transitions. The result shows ico is the
only stable shape of small clusters (Pd55-Pd309 using original EAM and Pd55
using optimized version) those are melting uniformly due to their small
diameter. An exception is cub Pd309 modeled via optimized EAM that transforms
to ico at elevated temperatures. A similar cub to ico transition was predicted
by original EAM for Pd923-Pd2075 clusters while for the larger clusters both
cub and ico are stable up to the melting point. As detected by G(r) and CNA,
moderate and large cub clusters were showing surface melting by nucleation of
the liquid phase at (100) planes and growth of liquid phase at the surface
before inward growth. While diagonal (one corner to another) melting was
dominating over ico clusters owing to their partitioned structure which
retarded the growth of the liquid phase. The large ico cluster, using optimized
EAM, presented a combination of surface and diagonal melting due to the
simultaneous diagonal melting started from different corners. Finally, the
melting temperature as well as latent heat of fusion were calculated and
compared with available models and previous studies which showed, unlike the
present result, the models failed to predict size-dependent motif crossover.
