On Harmonic Vibrational Frequency Convergence to the Complete Basis Set Limit in Water Dimers and Hydrogen Fluoride Dimers

Hardwick, Amanda (2014) On Harmonic Vibrational Frequency Convergence to the Complete Basis Set Limit in Water Dimers and Hydrogen Fluoride Dimers. Undergraduate thesis, under the direction of Gregory Tschumper from Chemistry & Biochemistry, The University of Mississippi.

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Abstract

This work focuses on the convergence of harmonic vibrational frequencies in the water dimer (H2O)2 and the hydrogen fluoride dimer (HF)2 to the complete basis set (CBS) limit. It also evaluates the effect of counterpoise (CP) correction procedures and the effect of sequentially removing diffuse functions from specific basis sets on the harmonic vibrational frequencies of these dimers. Computations were run using second-order Møller-Plesset perturbation theory (MP2) with large, flexible correlation consistent basis sets. This research aims to determine if removing diffuse functions can increase the speed of harmonic vibrational frequency computations without sacrificing accuracy. Results indicate that CP-correction procedures are not cost-effective. In order to get harmonic vibrational frequencies that deviate no more than 10 cm-1 from the CBS limit for these systems, a basis set of at least haQZ quality should be used. The average deviation in non-CP corrected frequencies using a TZ quality basis set from the CBS limit was 14 cm-1 basis set from the CBS limit was 17 cm-1 and CP-corrected frequencies using a QZ quality basis set from the CBS limit were both 7. CP-correction procedures are not cost-effective when basis sets of this size are used, however. Results also indicate that f and g diffuse functions can be removed from the oxygen. The average deviation in CP-corrected frequencies using a TZ quality. In contrast, these deviations in non-CP corrected and fluorine atoms in haQZ and aQZ basis sets—and also d and f diffuse functions from hydrogen in the aQZ basis set—without appreciably increasing deviations from the CBS limit frequencies.

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