The calculation of molecular vibrational frequencies has been investigated for a wide range of systems. Anharmonic vibrational frequencies calculated using second-order vibrational perturbation theory have been examined for a set of 88 small molecules (655 normal modes). The performance of different exchange-correlation functionals has been assessed, and anharmonic frequencies calculated using hybrid functionals are found to provide the best agreement with experiment. These functionals are not significantly improved by frequency scaling factors, indicating an absence of significant systematic error, and for the molecules studied the B97-1 and B97-2 functionals give the closest overall agreement with experiment. Experimental anharmonic shifts are found to be closest for the B3LYP and EDF2 functionals, highlighting the importance of an accurate harmonic force field, and investigations using modified hybrid functionals with increased fractions of Hartree-Fock exchange indicate that approximately 20 % is optimal. Partial Hessian harmonic vibrational analysis has been extended in order to calculate anharmonic frequencies, and the results show that this technique can be used to calculate accurate frequencies for vibrational modes that are spatially localized at a significantly reduced time cost. Several molecular systems are examined in order to demonstrate the effectiveness of this method, including organic molecules adsorbed on the Si(100)-2xl surface, microsolvated formamide, and the CH stretching region of polycyclic aromatic hydrocarbons.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:716493 |
| Date | January 2014 |
| Creators | Hanson-Heine, Magnus William Douglas |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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