>vector_potential</tt>) -- this option,
 in the current version, is a field constant in space, which permits us to describe
 an electric perturbation in the velocity gauge; (iv) an arbitrary scalar potential
 (<tt>scalar_potential</tt>).
<br><br>
 The "other descriptors" depend on which kind of external field has been indicated in
 the first column.
<br><br>
 (A) type = <tt>electric field, magnetic field, vector_potential</tt>
<br><br>
 For these cases, the syntax is:
<br><br>
 <tt>%TDExternalFields
 <br>&n