Accurate calculation of Franck-Condon factors for molecules with strongly anharmonic potentials

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The accurate calculation of vibronic transitions based on normal coordinates is currently limited to molecules with 3 or 4 atoms. The reason for this is the very demanding determination of reliable multi-dimensional potential energy surfaces, the corresponding correlated vibrational wave functions and theFranck-Condon (FC) factors themselves. The latter aspect is the primary subject of this proposal, but includes also the calculation of accurate potential energy surfaces based on explicitly correlated coupled-cluster methods. The focus is on molecules with up to 10 or 12 atoms, which show strongly anharmonic potential energy surfaces. An example comprises molecules with double well potentials, which may give rise to tunneling splittings. Anharmonic vibrational wave functions shall be determined by VSCF/VCI calculations (or modifications thereof). The size of the molecules as well as their strong anharmonicities require a number of new methods, which are subject of this proposal. This includes the efficient evaluation of FC integrals at the VSCF and VCI levels, the development of very fast vibration correlation methods and new prescreening techniques for anharmonic wave functions. The newly developed, fully automated program shall be used for systematic benchmark calculations, but also for molecules and molecular clusters of current interest.