ormed correctly) and preserves
 time-reversal symmetry (if the self-consistency problem is dealt with correctly).
 It is defined as:
<br><br>
 <MATH>
   U_{\rm EM}(t+\delta t, t) = \exp \left( -i\delta t H_{t+\delta t/2}\right)\,.
 </MATH>
</li>
<li><b>crank_nicholson</b>:  Classical Crank-Nicholson propagator.
<br><br>
 <MATH>
  (1 + i\delta t/2 H_{n+1/2}) \psi_{n+1} = (1 - i\delta t/2 H_{n+1/2}) \psi_{n}
 </MATH>
</li>
<li><b>crank_nicholson_sparskit</b>:  Classical Crank-Nicholson propagator. Requires th