Knowledge Bank

Due to advances in quantum mechnical methods over the last few years, it is now possible to determine ab initio potential energy surface in which fundamental vibrational frequencies are accurate to within $\pm 8cm-1$ on average, and molecular bond distances are accurate to within $\pm 0.001-0.003\backslash AA$, depending on the nature of the bond. That is, the potential energy surface have not been scaled or empirically adjusted in any way, showing that theoretical methods have progressed to the point of being useful in analyzing spectra that are not from a tighly controlled laboratory environment, such as rovibrational spectra from the interstellar medium. Some recent examples demonstrating this accuracy will be presented and discussed. These include the HNO, $CH4,C2H4$, and ClCN molecules is interesting due to the very large H-N anharmonicity, while ClCN has a very large Fermi resonance. The ab initio studies for the $CH4$ and $C2H4$ molecules present the first accurate full quartic force fields of any kind (i.e., whether theoretical or empirical) for a five-atom and six-atom system, respectively.