Intramolecular vibrational frequencies of water clusters (H2O)n (n=2-5):: Anharmonic analyses using potential functions based on the scaled hypersphere search method

Potential energy functions (PEFs) in the form of the sixth-order polynomial concerned with intramolecular motions of (H(2)O)(n) (n=2-5) were constructed by the scaled hypersphere search (SHS) based polynomial fitting (SHS-PF) method, in which the numbers of ab initio calculations can be considerably reduced via the selection of sampling directions by the SHS method. Concerning fundamental frequencies of the dimer cluster, the root mean square error from gas-phase experiments was only 10 cm(-1). For the trimer, tetramer, and pentamer, computed anharmonic frequencies reproduced available gas-phase experimental frequencies of HOH bending, donor-OH stretching, and free-OH stretching excellently with errors of approximately 10-20cm(-1) on average. It follows that the SHS-PF method is promising for highly accurate anharmonic vibrational analyses of intramolecular modes of H-bond clusters. (C) 2008 American Institute of Physics.