The performance of the semi-empirical AM1 method on small and nanometre-sized N2O clusters

Abstract

Large N₂O clusters containing up to 177 molecules were simulated using the semiempirical AM1 method. Simulated spectra of different cluster sizes show excellent agreement with experimental spectra. The vibrational band shape of the strong N–N stretching vibration is more strongly influenced by shape and size than that of the N–O stretching vibration. Stabilization energies and spectral band shapes confirm a crystal like structure of N₂O clusters generated in supersonic jet expansions. The AM1 results are carefully checked against experiment and high-level ab initio methods. Overall, AM1 reproduces experimental results (structure and vibrational frequencies) and MP2 dissociation energies of small N₂O clusters far better than any other quantum mechanical method studied here, although AM1 is not explicitly calibrated for the (N₂O)_n van der Waals system. The good performance of AM1 allows us to simulate N₂O clusters built from hundreds of molecules, a size range neither accessible by ab initio nor by continuum methods.