>: etrs
<br/><br> This variable determines which algorithm will be used to approximate
 the evolution operator <math>U(t+\delta t, t)</math>. That is, given
 <math>\psi(\tau)</math> and <math>H(\tau)</math> for <math>tau \le t</math>,
 calculate <math>t+\delta t</math>. Note that in general the Hamiltonian
 is not known at times in the interior of the interval <math>[t,t+\delta t]</math>.
 This is due to the self-consistent nature of the time-dependent Kohn-Sham problem:
 the Hamiltonian at a given time <ma