Advanced concepts in vibrational configuration interaction theory (VCI)

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The variational calculation of anharmonic vibrational frequencies of molecules and molecular clusters has gained considerable interest in the recent past. However, the methods employed face two computational bottlenecks, i.e. the accurate calculation of a multidimensional potential energy surface by sophisticated electronic structure methods and the account of vibration correlation effects. This proposal focuses on the latter aspect and new ideas are presented, which aim at the reduction of the computational cost and memory requirements as well as a balanced treatment of molecules with quasi-degenerate vibrational states. Essentially, a new configuration selective multistate vibrational configuration interaction program shall be developed, which allows for the balanced calculation of larger systems than the present code and which is not limited in any expansion orders of the correlation space and the representation of the potential energy surface. Once the new program is operative, it shall be applied to state-of-the-art problems in vibrational spectroscopy and the investigation of metal halide, (MX)_n, clusters with up to 20 atoms.