rrected Approximated Enforced Time-Reversal
 Symmetry (AETRS), this is the previous propagator but including
 a correction step to the exponential.
</li>
<li><b>etrs</b>:  The idea is to make use of time-reversal symmetry from the beginning:
<br><br>
 <MATH>
   \exp \left(-i\delta t/2 H_{n}\right)\psi_n = exp \left(i\delta t/2 H_{n+1}\right)\psi_{n+1},
 </MATH>
<br><br>
 and then invert to obtain:
<br><br>
 <MATH>
   \psi_{n+1} = \exp \left(-i\delta t/2 H_{n+1}\right) exp \left(-i\delta t/2 H_{n}\right)\psi