  The most important characteristic of glass transition is a jump in specific
heat $\Delta C_{p}$. Despite its significance, no standard theory exists to
describe it. In this study, first-principles molecular-dynamics (MD)
simulations are used to describe the glass transition of silica glass, which
presents many challenges. The novel view that state variables are extended to
include the equilibrium positions of atoms $\{ \bar{\bf R}_{j} \}$ is fully
used in analyzing the simulation results. Decomposing the internal energy into
three components (structural, phonon, and thermal expansion energies) reveals
that the jump $\Delta C_{p}$ of silica glass is entirely determined by the
component of structural energy. The reason for the small $\Delta C_{p}$ is its
high glass-transition temperature, which makes the fluctuation in the
structural energy insensitive to temperature changes. This significantly
affects how the Prigogine-Defay ratio $\it{\Pi}$ is interpreted, which was
previously unknown. The ratio $\it{\Pi}$ represents the ratio of the total
energy change to the contribution of thermal expansion energy at the glass
transition. The general property, $\it{\Pi}>1$, of glasses indicates that glass
transitions occur mainly by changes in the structural energy. Silica glass is
an extreme case in that the transition occurs entirely by the change in
internal structure, such as the distribution of the bending angle of Si--O--Si
bond.
