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 Title: 0.06 cm−1 DISCREPANCY FOR Li2→ 2Li AND 0.994 cm−1 FOR C → C+ BETWEEN LABORATORY AND COMPUTER SPECTROMETERS. Author(s): Dattani, Nikesh S. Subject(s): Mini-symposium: New Ways of Understanding Molecular Spectra Abstract: The energy at the empirical bond length of Li$_2(1^3\Sigma_u^+)$ of 4.1700\AA\,\footnote{Dattani N. S., Le$\,$Roy R. J. (2011) \textit{J. Mol. Spec.}. \textbf{268}, 199-210.; Semczuk M., \textit{et al.} (2013) \textit{Phys. Rev. A} \textbf{87}, 052505} was obtained at all-electron FCI level with an aug-cc-pCV5Z-NR basis set, all-electron CCSDT(Q) with aug-cc-pCV7Z-NR, and all-electron CCSD(T) with aug-cc-pCV8Z-NR; along with corrections due to special relativity converged with respect to electron correlation and basis set size using the spin-free Dirac-Coulomb Hamiltonian, and further such corrections at the Hartree-Fock level using the Breit and Gaunt Hamiltonians. Corrections to the point-size nucleus approximation were calculated but found to be negligible. The result was compared to the lowest energy of the best empirical potentials$^b$ with the empirical Born-Oppenheimer breakdown corrections removed, making it essentially an infinite-mass to infinite-mass comparison. The discrepancy between the energy obtained from laboratory spectroscopy and the energy obtained completely by the computer was only 0.06\,cm$^{-1}$, which is of the same order of magnitude as the uncertainty on the empirical value, which is $\pm$0.007\,cm$^{-1}$ before including the added uncertainty coming from the Born-Oppenheimer breakdown parameter $u_0$ which itself has an uncertainty of 0.01\,cm$^{-1}$. It is discussed what is necessary for the computer spectrometer to outperform the laboratory spectrometer. The ionization energy of the carbon atom was calculated at all-electron FCI level with aug-cc-pCV8Z-NR and aug-cc-pCV7Z-NR basis sets (the latter only for basis set extrapolation); along with corrections due to special relativity converged with respect to electron correlation and basis set size using the 1$e^{-}$ X2C Hamiltonian, further corrections using state-averaged Dirac-Fock for the contribution from the Breit Hamiltonian and some QED contributions; along with DBOC corrections to the clamped nucleus approximation converged with respect to electron correlation and basis set size. Again, corrections to the point-size nucleus approximation were calcualted but found to be negligible. The final energy was compared to the very recent experimental value published by NIST\footnote{Haris K., Krimada A. E., (2017) arXiv:1704.07474.} with the experimental spin-orbit lowering of 12.672508\,cm$^{-1}$ removed. The discrepancy was 0.994\,cm$^{-1}$ compared to the $\pm$0.009\,cm$^{-1}$ uncertainty in the laboratory value. Issue Date: 06/20/18 Publisher: International Symposium on Molecular Spectroscopy Citation Info: APS Genre: Conference Paper / Presentation Type: Text Language: English URI: http://hdl.handle.net/2142/100779 DOI: 10.15278/isms.2018.WB08 Other Identifier(s): WB08 Date Available in IDEALS: 2018-08-172018-12-12
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