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The total electronic energy ($E$) of the system changes as a function of the projectile position ($x$) since the projectile   (forced to maintain its velocity) deposits energy into the electronic system as it moves through the host atoms.   The increase of $E$ as a function of projectile displacement $x$ enables us to extract the electronic stopping power as a time-averaged quantity. quantity for each velocity,  \begin{equation}  S_\text{e} = \overline{\mathrm{d}E(x)/\mathrm{d}x}  \label{eq:stopping}   \end{equation}  $S_\text{e}$ has the dimension of a force (e.g. $E_\text{h}/a_0$) and it is has  the interpretation of a  drag force acting on the projectile.