e SPARSKIT library.
<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>magnus</b>:  Magnus Expansion (M4).
 This is the most sophisticated approach. It is a fourth-order scheme (a feature
 which it shares with the ST scheme; the other schemes are in principle second-order).
 It is tailored for making use of very large time steps, or equivalently,
 dealing with problem with very high-frequency time-dependence.
 It is still in a experimental 