Tracking the stilbene photoisomerization in the S(1) state using RASSCF.

In this work we compute the S1 potential energy curve responsible for stilbene cis-trans photoisomerisation employing the RASSCF approach, since the standard CASPT2//CASSCF protocol appears to be unsatisfactory in describing the stilbene S1 state. We find that RASSCF calculations, which are based on relatively few (but well chosen) configurations, produce qualitatively correct results and accurate relative excited state energies, both in the twisted and in the cis and trans regions of stilbene.