Vibrational dynamics anharmonic coupling parameters

Earlier theoretical work on the excited-state dynamics of pyrazine focused on the vibronically induced anharmonic couplings in the Si state, on the interactions between gerade and ungerade rm states or on selected PE functions and electron-vibrational coupling parameters. In a series of papers the Munich group has characterized the S2 and 51-state PE surfaces in the conformational subspace relevant for the short-time photophysics following the S2 — So and 5i — So transitions.In the course of this work, the treatment has steadily improved, as regards the accuracy of the electronic-structure calculation as well as the number and description of the vibrational modes considered. The most accurate calculations are based on the CASSCE/MRCI method with a basis set of DZP quality. 

Up to now, lattice dynamics calculations have addressed the translational phonons and the rotons separately. For the translational phonons one has used an isotropic (i.e. orientationally averaged) H2-H2 potential. The SCP method has been applied to the anharmonic vibrations [96], but it appeared to be necessary to introduce an approximate Jastrov function into this method (with one adjustable parameter) in order to obtain realistic results. The roton frequencies, and their softening at higher pressures (smaller volume) which precedes the disorder/order phase transition, have been calculated by the MF [97] and RPA [98] methods. Only quadrupole-quadrupole interactions were taken into account, and translation-rotation coupling was neglected. 

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