Calculation of IR frequencies and intensities in electrical and mechanical anharmonicity approximations: Application to small water clusters

We present a method for automatic computation of infrared (IR) intensities using parallel variational multiple window configuration interaction wave functions (P _ VMWCI2 algorithm). Inclusion of both mechanical and electrical anharmonic effects permits fundamental vibrational frequencies, including combinations and overtones, to be assigned. We use these developments to interpret the near-IR (NIR) and mid-IR (MIR) spectra of individual water clusters (H2O)(n) (n=1-4). Cyclic and linear systems are studied to provide equivalent reference theoretical data to investigate the structure of water as a function of density using NIR and MIR experimental spectra. Various density functional theory methods for generating the potential energy surface have been compared to reference results obtained at the CCSD(T) level [X. Huang et al., J. Chem. Phys. 128, 034312 (2008)]. For cyclic clusters, the IR intensities and frequencies obtained using BILYP/cc-pVTZ are found to be in very good agreement with the available experimental values and of the same orders of magnitude as the reference theoretical values. These data are completed by the vibrational study of linear systems. (C) 2010 American Institute of Physics. [doi:10.1063/1.3457482]