  Molecular dynamics simulation is employed to understand the thermodynamic
behavior of cuboctahedron (cub) and icosahedron (ico) nanoparticles with 2-20
number of shells (55-28741 atoms). The embedded atom method was used to
describe the interatomic potential. Conventional melting criteria such as
potential energy and specific heat capacity (C_p) caloric curves as well as
structure analysis by radial distribution function (G(r)) and common neighbor
analysis (CNA) were utilized simultaneously to provide a comprehensive picture
of the melting process. It is shown that the potential energy distribution and
surface energy (gamma_p) proposed here are holding several advantages over
previous criteria. In particular, potential energy distribution can distinguish
between interior and surface atoms and even corner, edge and plane atoms at the
surface. While G(r) and CNA are not surface sensitive methods and cannot
distinguish between surface melting and an allotropic transition. It is also
shown that allotropic change appears more clearly in C_p and gamma_p rather
than potential energy. However, determining accurate C_p requires enough
sampling to be averaged. Finally, a few issues in the current methods for
determining gamma_p were discussed and a simple method based on available
models was proposed which, independent of estimation of the surface area,
predicts the correct temperature and size-dependent trend in agreement with
Guggenheim-Katayama and Tolman's models, respectively.
