George A. Petersson
A density functional with dispersion correction terms is presented. The functional, APF, is comprised of 41.1 % B3PW91 and 58.9 % PBE1PBE, and the addition of a two-body dispersion term gives the APF-D density functional model. To account for anisotropy in the dispersion interaction a three body dispersion term is developed based on the Axilrod-Teller-Muto expression. A damping function for the three-body term was developed based on the two-body damping functions to attenuate the three-body correction at close internuclear distances. An updated version of APF-D, the APF-3D model, which includes the three-body correction is presented. In general, the APF-3D gives improved results over the APF-D, but further work is needed to optimize the model.
The geometry and frequency methods for the CBS-Wes1P compound model chemistry are presented. The geometry method is comprised of the APF-D/3D with the 3Za1Pa basis set. This method was designed to be as computationally inexpensive as possible while maintaining geometry accuracy of less than or equal to 0.01˚A. This rms error for the main group and alkaline earth elements is slightly above the target of 0.01˚A, while the transition and alkali metal rms errors are much larger than the target accuracy.
A frequency method using APF-D/3Za1Pa for the harmonic analysis followed by APF-D/2ZP0H for the anharmonic analysis is also presented. The method uses the second derivatives from the harmonic analysis for computation of the third and fourth derivatives for the anharmonic analysis. Other than for a few problematic cases, this method is superior to using scaled harmonic zero point energies. Further work is necessary to fine tune the dispersion interaction and the basis sets to achieve optimum performance of the geometry and frequency methods described here.
Dobek, Frank Julian, "Methods for Improved Molecular Geometry and Frequency Calculations" (2017). Dissertations. 81.
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