Highaccuracy ab initio water line intensities Lorenzo Lodi
High-accuracy ab initio water line intensities Lorenzo Lodi University College London Department of Physics & Astronomy
Talk summary • Variational methods for vibration-rotation spectra • Electronic structure treatment • Results for water line intensities
What we are aiming at? • Line intensities are extremely important. • They are also difficult to measure accurately. • Aim: accuracy of 1% (for majority lines). • We used the very best level of theory to achieve this goal. • Line intensities likely to be in error can be identified.
General scheme of solution • Born-Oppenheimer approximation. • Solve the electronic problem, obtaining Potential Energy Surface E(R) and Dipole Moment Surface m(R). • Use E(R) for the motion of the nuclei. • From m(R) and nuclear-motion wavefunctions calculate line intensities.
Solving for the nuclear motion • Molecule-fixed, J-dependent effective hamiltonians can be derived. • The resulting 3 -dimentional Schrödinger equation can be solved numerically. • An efficient technique to do this is the Discrete Variable Representation method. • Accuracy of line positions and intensities is limited by quality of PES and DMS.
Multi-reference methods • Standard methods such as DFT, MP 2 etc are not suitable. • Multi-reference methods are needed. • We used IC-MRCI+Q[8, 10], 6 Z basis set. • Relativistic effects accounted for.
Equilibrium water dipole From L. Lodi and J. Tennyson, J. Phys. B: At. Mol. Opt. Phys. 43, 133001 (2010)
Fitting the points • Intensities are very sensitive to oscillations in the DMS.
Lisak, Harvey and Hodges • D. Lisak, D. K. Harvey and J. T. Hodges, Phys. Rev. A 79, 052507 (2009) • 15 line intensities between 7170 -7183 cm-1 • Declared error of 0. 4%.
Lisak, Harvey and Hodges • 15 very accurate lines • One line where theory is consistently stronger by 30% • For the other 14 lines: average 1. 01(1)
Ponsardin & Browell • P. L. Ponsardin and E. V. Browell, J. Mol. Spectr. 185, 58 -70 (1997) • 40 line intensities around 12, 200 cm-1. • Declared error of 2%.
Summary comparison with HITRAN 2008 • • Total lines analysed: 26, 957 Stable lines: 19, 056 Unstable lines: 7, 901 Average of ratios for stable lines: 1. 00(4)
Final words • New high-accurate intensity linelist • Error of 1% in most cases • “Resonant” lines with large errors can be identified
- Slides: 20