  Individual cationic site--energies are explicitly determined from molecular
dynamics simulations of alkali silicate glasses, and the properties and
relevance of this local energetics to ion transport are studied. The absence of
relaxations on the timescale of ion transport proves the validity of a static
description of the energy landscape, as it is generally used in hopping models.
The Coulomb interaction among the cations turns out essential to obtain an
average energy landscape in agreement with typical simplified hopping models.
Strong correlations exist both between neighboring sites and between different
energetic contributions at one site, and they shape essential characteristics
of the energy landscape. A model energy landscape with a single vacancy is used
to demonstrate why average site--energies, including the full Coulomb
interaction, are still insufficient to describe the site population of ions, or
their dynamics. This model explains how the relationship between energetics and
ion dynamics is weakened, and thus establishes conclusively that a hopping
picture with static energies fails to capture all the relevant information. It
is therefore suggested that alternative simplified models of ion conduction are
needed.
