  We find that the density dependence of the glass transition temperature of
Lennard-Jones (LJ) and Weeks-Chandler-Andersen (WCA) systems can be predicted
from properties of the zero-temperature ($T=0$) glasses. Below a crossover
density $\rho_s$, LJ and WCA glasses show different structures, leading to
different vibrational properties and consequently making LJ glasses more stable
with higher glass transition temperatures than WCA ones. Above $\rho_s$,
structural and vibrational quantities of the $T=0$ glasses show scaling
collapse. From scaling relations and dimensional analysis, we predict a density
scaling of the glass transition temperature, in excellent agreement with
simulation results. We also propose an empirical expression of the glass
transition temperature using structural and vibrational properties of the $T=0$
glasses, which works well over a wide range of densities.
