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Exploring Electronically Excited States of HX Molecules: Ab Initio Calculations for HF and HCl

Delve into the world of electron excited states of HX molecules through ab initio calculations for HF and HCl. Explore methods like TD-DFT, CI, and CC for accurate results. Learn about challenges in handling non-Rydberg characters and bond breaking. Discover the significance of bases like cc-pvNz and aug-cc-pvNz for improved calculations. Dive into the complexities of HCl calculations and compare experimental results with ab initio findings to refine simulations. The future goal is to automate REMPI spectra simulation and enhance accuracy without the need for manual fitting processes.

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Exploring Electronically Excited States of HX Molecules: Ab Initio Calculations for HF and HCl

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  1. Ab initio REMPI Erlendur Jónsson

  2. MSc project • Electronically excited states of HX•••(H2O)n • After some trial calculations, this morphed into just calculations of HF and later on HCl

  3. Calculations • The calculations I’ve been using are all approximate methods of solving the Schrödinger equation

  4. Calculations • The excited-state calculations are apparently very hard. • The methods that are used for them are • TD-DFT • CI • CC

  5. TD-DFT • Time-dependent density functional theory • The cheapest method • Results are highly dependent on the selection of functional • Doesn’t handle non-Rydberg character properly

  6. CI • Configuration interaction • Handles correlated electrons • Can be formally exact • Extremely expensive • Common approximation is the CISD • Configuration interaction singles doubles

  7. CC • Coupled cluster • Can be formally exact like CI, but cheaper • CCSD(T) is currently the gold-standard of quantum chemistry

  8. CC • The S is singly excited electron • The D are double excited electrons • A parenthesis, like (T), means that triple excitations are partially calculated via pertubation • Implementations exist for up to CCSDTQPH

  9. CC - excited states • EOMCC • Equations of motions coupled cluster • Fairly reliable • A lot of research being done at the moment in new methods and extensions of the old methods

  10. CC • CC methods have a hard time handling bond breaking and high inter-nuclear distance • To compensate, new extensions have been added, such as the LR-CC and CR-CC (locally and completely renormalized)

  11. Bases • Systematic basis sets such as the cc-pvNz basis of Dunning, et al. give a very convenient way to improve calculations • But to handle very electronegative atoms, such as fluorine and chlorine, diffuse functions are needed in the basis which aren’t in the cc-pvNz so I’ve used the aug-cc-pvNz

  12. aug-cc-pvNz • Augmented correlation consistent polarized valence N zeta • N can be Double, Triple, Quadruple, 5 (quintuple) or 6 (sextuple) • Very popular for estimation of Complete Basis Set limit

  13. aug-cc-pcvNz • Extension of the aug-cc-pvNz where more core-core and core-valence correlation effects are added • When I tried excited triplet state calculations they proved to work considerably better than the aug-cc-pvNz

  14. HF • Was able to get fairly good results • The usual EOMCCSD calculations weren’t able to handle the V state of HF • Needed CR-EOM-CCSD(T) • But when that was achieved, the experimental setup didn’t work properly so I started calculations for HCl

  15. [1]K.A. Peterson and T.H. Dunning, J. Chem. Phys. 102, 2032,1995 [2] Retrieved from http://webbook.nist.gov [3] Bettendorff, M.,et al. Zeitschrift Fur Physik a-Hadrons and Nuclei, 304, 125-135, 1982

  16. HCl • Harder than HF • More electrons • Larger basis • I’ve used the experience gained from HF to progress further into the HCl calculations

  17. HCl • Is C∞v group, but the programs only offer C2v • This means that the excited state symmetries are a1, a2, b1 and b2 • Which aren’t the real symmetries which we have been seeking • So it hasn’t been easy finding what state is what in the resulting calculations

  18. HCl • Our hypothesis is that a1 states have Σ symmetry, a2 Δ symmetry and b1 have Π symmetry • b1 and b2 are degenerate

  19. Experimental vs. calculations • We of course need to compare the ab initio calculations to experimental results • The problems is that we have a potential curve

  20. Experimental vs. calculations • Currently we just fit the potential and get the various spectroscopic parameters • These parameters can then be used to simulate a REMPI spectra

  21. The future • Automate the simulation of the REMPI spectra and if possible remove the fitting part of method • Make a ab initio REMPI simulator

  22. Thank you for your attention

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