Characterization of theS1ndash;S2conical intersection in pyrazine usingabinitiomulticonfiguration selfhyphen;consistenthyphen;field and multireference configurationhyphen;interaction methods

摘要

Potentialhyphen;energy surfaces of the three lowest singlet states of pyrazine have been calculated as a function ofabinitiodetermined groundhyphen;state normal coordinates, using completehyphen;activehyphen;space selfhyphen;consistenthyphen;field (CASSCF) and multireference configuration interaction (MRCI) techniques. The conical intersection of theS1andS2adiabatic potentialhyphen;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. Transitionhyphen;dipolehyphen;moment functions have been obtained in the adiabatic and in the diabatic representation. The leading coefficients of the Taylor expansion of the diabatic potentialhyphen;energy and transitionhyphen;dipolehyphen;moment surfaces in terms of groundhyphen;state normal coordinates at the reference geometry have been obtained at the CASSCF/MRCI level. Using a vibronichyphen;coupling model Hamiltonian based on this Taylor expansion, the absorption spectrum of the interactingS1ndash;S2manifold has been calculated, taking account of the four spectroscopically most relevant modes.