aster.
 It is based on extrapolation of the time-dependent potentials. It is faster
 by about 40%.
<br><br>
 The only difference is the procedure to estimate @math{H_{n+1}}: in this case
 it is extrapolated via a second-order polynomial by making use of the
 Hamiltonian at time @math{t-2\delta t}, @math{t-\delta t} and @math{t}.
</li>
<li><b>exp_mid</b>:  Exponential Midpoint Rule (EM).
 This is maybe the simplest method, but it is very well grounded theoretically:
 it is unitary (if the exponential is perf