_{n}.
 </MATH>
<br><br>
 But we need to know <math>H_{n+1}</math>, which can only be known exactly through the solution
 <math>\psi_{n+1}</math>. What we do is to estimate it by performing a single exponential:
 <math>\psi^{*}_{n+1}=\exp \left( -i\delta t H_{n} \right) \psi_n</math>, and then
 <math>H_{n+1} = H[\psi^{*}_{n+1}]</math>. Thus no extrapolation is performed in this case.
</li>
<li><b>aetrs</b>:  Approximated Enforced Time-Reversal Symmetry (AETRS).
 A modification of previous method to make it f