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Klaus Schulten, Uri Dinur, and Barry Honig. The spectra of carbonium ions, cyanine dyes, and protonated Schiff base polyenes. Journal of Chemical Physics, 73:3927-3935, 1980. The origin of the strong bathochromic shift of the visual pigment supposedly lies in the carbonium ion character of the protonated Schiff base retinal. Following this conjecture, we have carried out a comparative study of the spectra of the carbonium ions, cyanine dyes, and protonated Schiff base polyenes. In our calculation, we have employed PPP (for the carbonium ions) and CNDO/S parametrized Hamiltonians. For a correct description of the electronic excitations in these linear $\pi$-systems, we have included all single, double, and in some cases also all triple and quadruple excited configurations in the CI expansion. Our study revealed a close resemblance of the spectral behavior of these three compounds. In particular, we found that higher excitations, accounting for the effect of electron correlation, induce a strong red shift of the main ($S_1$$\leftarrow$$S_0$) absorption band. This behavior is due to the mainly covalent character of the $S_1$ state which is poorly described by single excited configurations. As in the polyenes, there exists also a weakly absorbing state of double excited character which in contrast to the polyene case lies above the allowed state. Full Text

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