  This paper presents an $ab$ $initio$ study of hydrogen dynamics inside
nanometer-size voids in $a$-Si within the framework of the density-functional
theory for a varying hydrogen load of 10 to 30 H atoms/void at the low and high
temperature of 400 K and 700 K, respectively. Using the local density
approximation and its generalized-gradient counterpart, the dynamics of
hydrogen atoms inside the voids are examined with an emphasis on the diffusion
of H atoms/molecules, and the resulting nanostructural changes of the void
surfaces. The results from simulations suggest that the microstructure of the
hydrogen distribution on the void surfaces and the morphology of the voids are
characterized by the presence of a significant number of monohydride Si-H
bonds, along with a few dihydride Si-H$_2$ configurations. The study also
reveals that a considerable number of (about 10--45 at.%) total H atoms inside
voids can appear as H$_2$ molecules for a hydrogen load of 10--30 H atoms/void.
The approximate shape of the voids is addressed from a knowledge of the
positions of the void-surface atoms using the convex-hull approximation and the
Gaussian broadening of the pseudo-atomic surfaces of Si and H atoms.
