Dynamics of radiationless transitions in large molecular systems: A Franck–Condon-based method accounting for displacements and rotations of all the normal coordinates

An efficient method to study the dynamics of radiationless transition in large molecular systems is proposed. It is based on the use of the whole set of normal coordinates of vibration and allows for taking properly into account both the displacements and the mix of the normal modes upon transition between two electronic states. The Hamiltonian matrix elements are written in terms of generalized Franck–Condon integrals and are analytically evaluated by recursion formulas. Applications to the S2→S1 internal conversion in pyrazine and to long-range electron transfer between quinones in photosynthetic reaction centers are given.