one of the function names
 given in the first column of the <tt>TDFunctions</tt> block.
<br><br>
 (B) type = <tt>scalar_potential</tt>
<br><br>
 <tt>%TDExternalFields
 <br>&nbsp;&nbsp; scalar_potential | "scalar_expression" | freq | envelope_function_name
 <br>%</tt>
<br><br>
 The scalar potential is not just a dipole, but any expression given by the string
 "scalar_expression". The temporal shape is determined by the envelope function
 defined by <tt>envelope_function_name</tt>.
<br><br>
 A NOTE ON UNITS:
