Two-State Three-Mode Parametrization of the Force Field of a Retinal Chromophore Model.

In recent years the potential energy surfaces of the penta-2,4-dieniminium cation have been investigated using several electronic structure methods. The resulting pool of geometrical, electronic and energy data, provides the basis for the construction of a topographically correct analytical model of the molecule force field and, therefore, for a better understanding of these systems which include the chromophore of visual pigments. In the present contribution, we report the construction of such a model for regions of the force field driving the photochemical and thermal isomerization of the central double bound of the cation. It is shown that the proposed and corresponding set of parameters are able to reproduced the complex topographical and electronic structure features seen in the electronic correlated data obtain at the XMCQDPT2//CASSCF/6-31G* level of theory.