Anharmonic Frequencies for the Detection of Large Molecules in Space

Quartic force fields (QFFs) offer highly accurate anharmonic rovibrational spectroscopic constants. The state-of-the-art composite approach is known as CcCR and often achieves accuracies within 1 cm^-1 of gas-phase experiment for fundamental frequencies and 20 MHz for principle rotational constants. The performance of explicitly-correlated methods is nearly as good, offering accuracies of 5-7 cm^-1 at a much lower computational cost. However, both of these QFF approaches typically rely on a complex internal coordinate system that can be difficult to derive for large or highly symmetric molecules. Extending this methodology to use generic Cartesian coordinates and the analytic derivatives found in many popular quantum chemistry packages will allow the elucidation of such spectral data for larger molecules. Preliminary work on ammonia borane and other even larger molecules demonstrates the efficacy of this new approach.