| Summary: | The procedure for calculating core-electron binding energies (CEBEs),
based on the unrestricted generalized transition state (uGTS) model combined
with density functional theory (DFT) employing Becke's 1988 exchange
(B88) and Perdew's 1986 correlation (P86) functionals, which has proven to
yield highly accurate results for C, N, O, and F cases, was extended to boroncontaining
molecules and to Si, P, S, CI, and Ar cases.
Both unsealed and scaled basis sets were used in the studies of boroncontaining
molecules. The scaled-pVTZ basis set was as highly efficient for
boron as it had been found to be for C, N, O, and F cases; the average
absolute deviation (AAD) of the calculated CEBEs from experiment was
0.24 eV, compared to 0.23 eV for the much larger cc-pV5Z basis set. A
generalization of the exponent-scaling methodology was proposed and tested
on boron-containing molecules, and was found not to improve the original
results to a significant extent.
The preliminary calculations of Si, P, S, CI, and Ar CEBEs indicated
that, in order to achieve the accuracy obtained for second-period elements,
refinement of the basis sets and inclusion of relativistic effects are necessary.
As an additional application of the DFT/uGTS/scaled-pVTZ approach,
the CEBEs of four isomers of C₃H₅NO were calculated. The distinctive nature
of the core-ionization spectra of the isomers was depicted by the results,
thus illustrating the potential utilization of accurate theoretical predictions as a complement to electron spectroscopy for chemical analysis.
The model error in uGTS calculations and the errors in the functionals
employed were calculated. It was observed that the high accuracy of the
B88/P86 combination was due to a fortuitous cancellation of the functional
and model errors. In view of this finding, a Kohn-Sham total-energy difference
approach, which eliminates the model error, was investigated.
Ten functional combinations and several basis sets (including unsealed,
scaled, and core-valence correlated functions) were tested using a database
of reliable observed CEBEs. The functionals designed by Perdew and Wang
(1986 exchange and 1991 correlation) were found to give the best performance
with an A A D from experiment of 0.15 eV. The scaled basis sets did not
perform as well as they did in the uGTS calculations, but it was found
that the core-valence correlated cc-pCVTZ basis functions were an excellent
alternative to the cc-pV5Z set as they provided equally accurate results and
could be applied to larger molecules.
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