  This paper provides collapses of massive, fully convective, and non-rotating
white dwarfs (WDs) formed by accretion-induced collapse or merger-induced
collapse and the subsequent explosions with the general relativistic
neutrino-radiation hydrodynamics simulations. We produce initial WDs in
hydrostatic equilibrium, which have super-Chandrasekhar mass and are about to
collapse. The WDs have masses of 1.6$M_\odot$ with different initial central
densities specifically at $10^{10}$, $10^{9.6}$, $10^{9.3}$ and $10^{9.0}\,{\rm
g\,cm^{-3}}$. First, we check whether initial WDs are stable without weak
interactions. Second, we calculate the collapse of WDs with weak interactions.
We employ hydrodynamics simulations with Newtonian gravity in the first and
second steps. Third, we calculate the formation of neutron stars and
accompanying explosions with general relativistic simulations. As a result, WDs
with the highest density of $10^{10}\,{\rm g\,cm^{-3}}$ collapse not by weak
interactions but by the photodissociation of the iron, and three WDs with low
central densities collapse by the electron capture as expected at the second
step and succeed in the explosion with a small explosion energy of $\sim
10^{48}$ erg at the third step. By changing the surrounding environment of WDs,
we find that there is a minimum value of ejecta masses being $\sim
10^{-5}M_{\odot}$. With the most elaborate simulations of this kind so far, the
value is one to two orders of magnitude smaller than previously reported values
and is compatible with the estimated ejecta mass from FRB~121102.
