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Towards a Spectroscopically Flexible Water Dimer Potential Energy Surface

Towards a Spectroscopically Flexible Water Dimer Potential Energy Surface. Imperial College, December 2008. Ross E. A. Kelly , and Jonathan Tennyson Department of Physics & Astronomy University College London Gerrit C. Groenenboom and Ad van der Avoird

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Towards a Spectroscopically Flexible Water Dimer Potential Energy Surface

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  1. Towards a Spectroscopically Flexible Water Dimer Potential Energy Surface Imperial College, December 2008 Ross E. A. Kelly, and Jonathan Tennyson Department of Physics & Astronomy University College London Gerrit C. Groenenboom and Ad van der Avoird Theoretical Chemistry, Institute for Molecules and Materials, Radboud University, Nijmegen.

  2. Outline • I. Motivations • II. The Water Dimer • III. Vibration-Rotation Tunnelling • IV. Dimer Potentials • V. Theoretical Method • VI. VRT States • VII. Monomer Band Origins • VIII. Monomer Corrected Surface • IX. Conclusions & Further Work

  3. I. Motivations • to understand water dimer absorption throughout visible and IR region in the atmosphere. • To create a high accuracy water dimer spectra in agreement with experiments. • To create a linelist of all possible water dimer transitions.

  4. II. Water Dimer • To get all Vibration Rotation Energy Levels Calculations require: • 1. Some theoretical methodology for solving the Hamiltonian • 2. High accuracy Potential Energy Surface • Preferably fully flexible (12D) • 4 O-H distances • 2 H-O-H Angles • O-O distance • 5 Euler Angles

  5. II. Water Dimer X. Huang, B. J. Braams, J. M. Bowman, J. Phys. Chem. A110, 445 (2006). • Extremely Complex Landscape

  6. III. Vibration-Rotation Tunnelling • Complicated further by tunnelling effects! • Tunnelling between eqiulavent states in the PES is feasible! • Acceptor Tunnelling: • No bond breaking here • Lowest tunnelling barrier • Also, by breaking the Hydrogen bond, other tunnelling paths possible: • Donor-Acceptor interchange • Donor Bifurcation Tunnelling

  7. III. Labelling Water Dimer States • Can be represented by Permutation-Inversion Group G16. 2 2 6 6 4 3 1 1 5 5 3 4 1 1 2 6 2 6 3 4 5 5 4 3 1 2 1 2 6 6 3 3 5 5 4 4 2 2 1 1 • Isomorphic to D4h withIrreducible Elements: A1+, A2+, A1-, A2-, B1+, B2+, B1-, B2-, E+, E- -> Water Dimer Spectroscopic Labels 6 6 4 4 5 5 3 3

  8. IV. Dimer Potentials Available • Many PES’s available! • Benchmark: • 6DCC-pol Potential [1] - Rigid Monomers • New (2) 12D PESs by Huang, Braams & Bowman [2,3] • HBB0 [2] • and HBB [3] – NEW 12D PES - All completely ab initio - Good agreement with experiment [1] R. Bukowski, K. Szalewicz, G. C. Groenenboom, A. van der Avoird, Science315, p1249-1252 (2007). [2] X. Huang, B. J. Braams, J. M. Bowman, J. Phys. Chem. A110, 445 (2006). [3] X. Huang, B. J. Braams, J. M. Bowman, R. E. A. Kelly, J. Tennyson, G. C. Groenenboom, A. van der Avoird, J. Chem. Phys. 128, 034312 (2008)

  9. 30 000 configurations. • Calculated at coupled-cluster, single and double and perturbative treatment of triple excitations method. • augmented, correlation consistent, polarized triple zeta basis set. • Polynomial fit with 5227 coefficients. • However, compared to CCpol potential (benchmark): • Lower relative grid coverage of the surface than benchmark 6D PES. • Not extrapolated to the complete basis set limit (CBS) • No bond functions in analytical fit • Dissociation less accurately described IV. HBB PES for H4O2 12D [1] X. Huang, B. J. Braams, J. M. Bowman, R. E. A. Kelly, J. Tennyson, G. C. Groenenboom, A. van der Avoird, J. Chem. Phys. 128, 034312 (2008).

  10. IV. HBB PES for H4O2 • Dissociation Energy = 1665.82 cm-1 / 1757.90 cm-1 • Other tests? • Compare with Low temperature high-resolution Tetrahertz Spectroscopy (prepared in supersonic molecular beams), around 5 K. • How can this be done theoretically?

  11. V. Theoretical Method for VRT levels • Rigid monomer Hamiltonian [1]: • Only for the Intermolecular modes • Used for water dimer previously, detailed account [2] • Coupled product of Symmetric rotor functions (Wigner-D functions) for the Angular coordinates • Radial basis: sinc Discrete Variable Representation (DVR) [1] G. Brocks, A. van der Avoird, B. T. Sutcliffe, J. Tennyson, Mol. Phys. 50, 1025 (1983). [2] G. C. Groenenboom, et al., JCP 113, 6702 (2000).

  12. VI. Ground State VRT states for H4O2 • Very good agreement with: • Ground State Tunnelling splittings • Rotational Constants • Not so good agreement with: • Acceptor Tunnelling [1] X. Huang, B. J. Braams, J. M. Bowman, R. E. A. Kelly, J. Tennyson, G. C. Groenenboom, A. van der Avoird, J. Chem. Phys. 128, 034312 (2008).

  13. VI. Ground State VRT for D4O2 • Excellent agreement with: • Ground State Tunnelling splittings • Rotational Constants • Not so good agreement with: • Acceptor Tunnelling [1] X. Huang, B. J. Braams, J. M. Bowman, R. E. A. Kelly, J. Tennyson, G. C. Groenenboom, A. van der Avoird, J. Chem. Phys. 128, 034312 (2008).

  14. VI. More low level VRT States for H4O2 • In cm-1 • Red – ab initio potential • Black – experimental • GS – ground state • DT – donor torsion • AW – acceptor wag • AT – acceptor twist • DT2 – donor torsion overtone

  15. VI. More low level VRT States for D4O2 • In cm-1 • Red – ab initio potential • Black – experimental • GS – ground state • DT – donor torsion • AW – acceptor wag • AT – acceptor twist • DT2 – donor torsion overtone

  16. Fix Excite VII. Monomer Band Origins • New 12D Huang et al. PES seems to work well: • for low-level dimer VRT states • How about for H2O monomer energy levels? • Use DVR3D [1] for Water monomer levels: 100 bohr 1. J. Tennyson et al., Comp. Phys. Comm. 2004, 163, 85-116. 1. J. Tennyson et al., Comp. Phys. Comm. 2004, 163, 85-116.

  17. VII. Monomer Band Origins • Comparison is not so good. • Blue HBB dimer • Green HBB0 • Red Shirin 2008. • (Monomer).

  18. VIII. Adding Monomer Correction • Correction for monomer modes: • New Potential Expression: • Tests for Potential • Evaluation of the saddle points. • Evaluation of the monomer & dimer VRT states.

  19. VIII. Monomer Corrected Surface Not Particularly worse than HBB potential.

  20. VIII. Ground State VRT levels for H4O2 • Still a very good agreement with exp. • Nothing is changed significantly.

  21. VIII. More VRT States for HBB+MCsurface • Again, agreement is not significantly worse.

  22. IX. Conclusions & Further Work • Introduction of monomer correction makes PES more transferable for spectroscopic purposes. • Little effect on dimer characteristics. • Monomer band origins significantly improved. • We are working towards a model which incorporates monomer excitations into the dimer states, so spectra can be produced.

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