“…In free clusters, enhanced surface energy and the concomitant increase in the surface stress causes a lattice contraction at the NP surface, softening of interatomic force constants, and a suppression of D and melting temperatures. 1 On the other hand, the increased D observed for certain systems has been attributed to structural stiffening and inhomogeneous internal stress correlated with the NP size, surroundings ͑e.g., adsorbates/ surface ligands, 15,16,19 a matrix encapsulating the NPs or a support 12,13 ͒, and to the presence of structural defects and multiple grains in large NPs. 14 It is evident that the understanding of thermal, structural, and electronic properties of these systems is hindered by their complexity, due to the multiple competing factors that define their behavior, and any effort in sorting out these influences will be helpful for developing first-principles theories.…”