Calculation of High-Resolution IR Spectra, Including Line Lists and Intensities, for Astronomically Interesting Molecules and their Isotopes: NH3, CO2, SO2, CH3OH, CH3CN, and CH3-O-CH3
Determination of highly accurate potential energy surfaces (PES) and dipole moment surfaces (DMS) for ammonia (NH3), carbon dioxide (CO2), sulfur dioxide (SO2), methanol (CH3OH), acetonitrile (CH3CN), and dimethyl ether (CH3-O-CH3, DME), which are isotope independent by using high-level electronic structure calculations, including core correlation, scalar relativistic corrections, basis set extrapolation, and higher-order electron correlation corrections. This determination is followed by inclusion of a mass-dependent diagonal Born-Oppenheimer correction and refinement with the available experimental data in order to obtain the most accurate PES result possible. The highly accurate PES and DMS results are used to compute line positions and dipole moment matrix elements (for intensities) for eigenstates up to 20,000 cm-1 for SO2/CO2 10,000 cm-1 for NH3 and 5,000 cm-1 for CH3OH / CH3CN / DME (above the zero-point level). Finally, we expect these line positions and intensities to be of a similar quality or better than the previous H2O and recent NH3 calculations [21,23], i.e. 0.03 - 0.05 cm-1. The highly accurate line lists can be used to analyze astronomical spectra from NASA missions and other astronomical observations. It is evident from our introductory discussions that highly accurate knowledge of their spectra will be critical to the success of the Herschel, SOFIA, and JWST missions. Techniques and experiences developed in this work will also be used in obtaining highly accurate line lists for other astronomical molecules in the future.