th>\tau</math> is built from the
 "solution" wavefunctions at that time.
<br><br>
 Some methods, however, do require the knowledge of the Hamiltonian at some
 point of the interval <math>[t,t+\delta t]</math>. This problem is solved by making
 use of extrapolation: given a number <math>l</math> of time steps previous to time
 <math>t</math>, this information is used to build the Hamiltonian at arbitrary times
 within <math>[t,t+\delta t]</math>. To be fully precise, one should then proceed
 <i>self-consiste