>ftchd</tt> is created (it contains the Fourier transform of the charge density
 at each time). The type of the applied external field can be set by
 an optional last number. Possible options are <tt>qexp</tt> (default), </tt>qcos</tt>,
 <tt>qsin</tt>, or <tt>qcos+qsin</tt>.

<br/><i>Options</i>:
<ul>
<li><b>qexp</b>:  External field is exp(<i>iq.r</i>).
</li>
<li><b>qcos</b>:  External field is cos(<i>q.r</i>).
</li>
<li><b>qsin</b>:  External field is sin(<i>q.r</i>).
</li>
<li><b>qbessel</b>:  External f