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Ab initio REMPI

Ab initio REMPI. Erlendur J ónsson. MSc project. Electronically excited states of HX•••(H 2 O) n After some trial calculations, this morphed into just calculations of HF and later on HCl. Calculations.

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Ab initio REMPI

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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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