ev polynomials:
<br><br>
 <MATH>\exp_{\rm CHEB} \left( -i\delta t H \right) = \sum_{k=0}^{\infty} (2-\delta_{k0})(-i)^{k}J_k(\delta t) T_k(H),</MATH>
<br><br>
 where <math>J_k</math> are the Bessel functions of the first kind, and H has to be previously
 scaled to <math>[-1,1]</math>.
 See H. Tal-Ezer and R. Kosloff, J. Chem. Phys. <b>81</b>,
 3967 (1984); R. Kosloff, Annu. Rev. Phys. Chem. <b>45</b>, 145 (1994);
 C. W. Clenshaw, MTAC <b>9</b>, 118 (1955).
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