  We use a swap Monte Carlo algorithm to numerically prepare bulk glasses with
kinetic stability comparable to that of glass films produced experimentally by
physical vapor deposition. By melting these systems into the liquid state, we
show that some of our glasses retain their amorphous structures longer than
10^5 times the equilibrium structural relaxation time. This exceptional kinetic
stability cannot be achieved experimentally for bulk materials. We perform
simulations at both constant volume and constant pressure to demonstrate that
the density mismatch between the ultrastable glass and the equilibrium liquid
accounts for a major part of the observed kinetic stability.
