Vibrational spectra of ammonia clusters from n=3 to 18

We have calculated the vibrational spectra of the umbrella (nu (2)), the sy mmetric (nu (1)), and the asymmetric (nu (3)) N-H stretch mode for ammonia clusters from n=3-18. The results are based on recent structure calculation s and a molecular perturbation approach that includes the anharmonicities. Clusters with high symmetry exhibit few lines only and show the expected bl ue shifts for the umbrella and red shifts for the two N-H stretch modes. Th e calculated frequencies of the umbrella mode agree very well with experime ntal results for n=2-5 as far as the general shape is concerned, but overes timate the absolute shifts. (C) 2001 American Institute of Physics.