Characterization of the S1–S2 conical intersection in pyrazine using ab initio multiconfiguration self-consistent-field and multireference configuration-interaction methods
Potential‐energy surfaces of the three lowest singlet states of pyrazine have been calculated as a function of ab initio determined ground‐state normal coordinates, using complete‐active‐space self‐consistent‐field (CASSCF) and multireference configuration interaction (MRCI) techniques. The conical intersection of the S1 and S2 adiabatic potential‐energy surfaces has been mapped out in selected subspaces spanned by the most relevant vibrational coordinates. A unitary transformation from the adiabatic to a quasidiabatic electronic representation is performed, which eliminates the rapid variations of the wave functions responsible for the singularity of the nonadiabatic coupling element. Transition‐dipole‐moment functions have been obtained in the adiabatic and in the diabatic representation. The leading coefficients of the Taylor expansion of the diabatic potential‐energy and transition‐dipole‐moment surfaces in terms of ground‐state normal coordinates at the reference geometry have been obtained at the CASSCF/MRCI level. Using a vibronic‐coupling model Hamiltonian based on this Taylor expansion, the absorption spectrum of the interacting S1–S2 manifold has been calculated, taking account of the four spectroscopically most relevant modes.