ul>
<li><b>electric_field</b>:  The external field is an electric field, the usual case when we want to describe a
 laser in the length gauge.
</li>
<li><b>magnetic_field</b>:  The external field is a (homogeneous) time-dependent magnetic field.
</li>
<li><b>vector_potential</b>:  The external field is a time-dependent homogeneous vector potential, which may describe
 a laser field in the velocity gauge.
</li>
<li><b>scalar_potential</b>:  The external field is an arbitrary scalar potential, which may descr