Momentum-transfer vector $\vec{q}$ to be used when calculating matrix elements
$\left< f \left| e^{i \vec{q} \cdot \vec{r}} \right| i \right>$.
This enables the calculation of the dynamical structure factor,
which is closely related to generalized oscillator strengths.
If the vector is not given, but TPA output is requested (Output = TPA),
only the oscillator strengths are written in the output file.
For example, to use $\vec{q}$ = (0.1, 0.2, 0.3), set