Vibrational spectroscopy of large clusters

Calculating vibrational spectroscopy of large anharmonic polyatomic system is a major challenge in the field of computational chemistry. In this work, we have developed a powerful method to obtain vibrational spectroscopy and have applied it to problems of major interest such as water clusters, (H{dollar}sb2{dollar}O){dollar}sb{lcub}rm n{rcub}{dollar}, and halide anion-water clusters, X{dollar}sp-{dollar}(H{dollar}sb2{dollar}O){dollar}sb{lcub}rm n{rcub}.{dollar} As a result, understanding is gained of spectroscopy and physical properties of hydrogen bonded neutral and ionic systems. Furthermore, major improvements are made in the potential functions of these systems via contact with experiments. Some of the important findings are: (1) VSCF-CC method is a powerful and effective tool to obtain vibrational energy levels and wavefunctions for large anharmonic clusters on a full quantum mechanical footing. (2) First all mode vibrational spectrum calculation of water clusters, (H{dollar}sb2{dollar}O){dollar}sb{lcub}rm n{rcub}{dollar}, up to n = 8 have been calculated and physical insights were gained. The results throw light on the properties of the spectrum, especially with respect to collective shearing and torsional modes. (3) The VSCF-CC method makes possible high quality tests of the leading empirical potential functions. All available potential functions for (H{dollar}sb2{dollar}O){dollar}sb{lcub}rm n{rcub}{dollar} and X{dollar}sp-{dollar}(H{dollar}sb2{dollar}O){dollar}sb{lcub}rm n{rcub}{dollar} failed at the spectroscopic level. The VSCF-CC method allows, for the first time, fitting of much improved empirical potential for large anharmonic polyatomic systems against the available experimental data. (4) The accurate vibrational eigenstates computed by VSCF-CC method can be used as initial states for dynamical process in many systems. Thus UV absorption simulations were carried out for water clusters (H{dollar}sb2{dollar}O){dollar}sb{lcub}rm n{rcub}{dollar}, n = 2-6. Very interesting results were determined on the absorption lineshapes as a function of cluster size, and also on photochemical dynamics following the absorption process. (5) The role of hydrogen bonding cooperative effects on neutral water clusters and halide anion-water clusters as a function of cluster size were explored. This result makes important contact with the main thrust of cluster studies: Bridging the gap between the isolated molecules and bulk properties. Overall, the sensitivity and the usefulness of the VSCF-CC method to study the large anharmonic clusters will be demonstrated.