The polarization behavior of the low lying excited states in the vicinity of the perpendicularly twisted (D2d) ethylene has been investigated in a quantum mechanical CISD approach, in which the quantum system was embedded in a polarized dielectric continuum modeling a non-symmetrical distribution of the solvent around the solute. The results show a strong polarization in the two lowest lying excited states in the region where the two vacuum energy surfaces of those states intersect, which strongly suggests that the lowering of the symmetry of the solvent shell can provide the adiabatic coupling for the avoided crossing between both potential energy surfaces. All examined values of ∈, with the exception of the smallest investigated value (∈ = 2.0), showed the polarization to keep intact on progressive twisting towards the perpendicular geometry.
Zijlstra, R. W. J., Van Duijnen, P. T., & De Vries, A. H. (1996). Polarization of the excited states of twisted ethylene in a non-symmetrical environment. Chemical Physics, 204(2-3 SPEC. ISS.), 439–446. https://doi.org/10.1016/0301-0104(95)00426-2