  We systematic investigate the collapse of a set of open-cell nanoporous Cu
(np-Cu) with the same porosity and shapes, but different specific surface area,
during thermal annealing, via performing large-scale molecular dynamics
simulations. Surface premelting is dominated in their collapses, and surface
premelting temperatures reduce linearly with the increase of specific surface
area. The collapse mechanisms are different for np-Cu with different specific
surface area. If the specific surface area less than a critical value ($\sim$
2.38 nm$^{-1}$), direct surface premelting, giving rise to the transition of
ligaments from solid to liquid states, is the cause to facilitate falling-down
of np-Cu during thermal annealing. While surface premelting and following
recrystallization, accelerating the sloughing of ligaments and annihilation of
pores, is the other mechanism, as exceeding the critical specific surface area.
The recrystallization occurs at the temperatures below supercooling, where
liquid is instable and instantaneous. Thermal-induced surface reconstruction
prompts surface premelting via facilitating local "disordering" and "chaotic"
at the surface, which are the preferred sites for surface premelting.
