Vibrational spectra of ammonia clusters from n=3 to 18

We have calculated the vibrational spectra of the umbrella (nu (2)), the symmetric (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 calculations and a molecular perturbation approach that includes the anharmonicities. Clusters with high symmetry exhibit few lines only and show the expected blue shifts for the umbrella and red shifts for the two N-H stretch modes. The calculated frequencies of the umbrella mode agree very well with experimental results for n=2-5 as far as the general shape is concerned, but overestimate the absolute shifts. (C) 2001 American Institute of Physics.