i
 subspace. If the maximum dimension allowed by <tt>TDExpOrder</tt> is not
 enough to meet the criterion, the above-mentioned warning is emitted.
</li>
<li><b>taylor</b>:  This method amounts to a straightforward application of the definition of
 the exponential of an operator, in terms of its Taylor expansion.
<br><br>
 <MATH>\exp_{\rm STD} (-i\delta t H) = \sum_{i=0}^{k} {(-i\delta t)^i\over{i!}} H^i.</MATH>
<br><br>
 The order <i>k</i> is determined by variable <i>TDExpOder</i>.
 Some numerical consider