Importance of Monomer-Flexibility Effects for Spectra of Molecular Clusters

A global full-dimensional description of interactions in a molecular van der Waals cluster, including both inter and intramolecular degrees of freedom, may seem to be the necessary starting point for high-accuracy nuclear dynamics calculations. Such calculations are currently able to make predictions for clusters, molecular collisions, and condensed phases accurate enough to be confronted with experiment. However, the all-dimensional treatment becomes prohibitively expensive for clusters with more than 6 atoms due to the "curse of dimensionality". On the other hand, the rigid-monomer approximation allows applications to much larger clusters. We show on the example of H₂-CO that if the rigidity is imposed via averaging over monomer vibrations, the predictions from such a reduced-dimensionality model can be about as accurate as those from the full-dimensional one; in fact, here both models predict spectra equally well. Moreover, we show that an approximate version of such an averaged surface, based on the Taylor expansion, which does not require the development of a full-dimensional surface and is affordable for larger molecules, also works very well. In contrast, models based on frozen geometries of monomers work much worse. Spectral and scattering calculations with the vibrationally averaged reduced-dimensionality models will result in insights into soft condensed matter properties, cold and ultracold molecular collisions, and physics of cold interstellar clouds that are currently not possible.