ations (by Jeff Giansiracusa and George F. Bertsch;
 see http://www.phys.washington.edu/~bertsch/num3.ps)
 suggest the 4th order as especially suitable and stable.
</li>
<li><b>chebyshev</b>:  In principle, the Chebyshev expansion
 of the exponential represents it more accurately than the canonical or standard expansion.
 As in the latter case, <tt>TDExpOrder</tt> determines the order of the expansion.
<br><br>
 There exists a closed analytic form for the coefficients of the exponential in terms
 of Chebysh