Selection of the Calculation Method

The Density Functional method or commonly called the DFT method is, as mentioned in chapter 2, one with the lowest computational costs though its similarity in many ways to ab initio methods. DFT has the big advantage to be usable for large systems where high level computations are costly or not possible. In this section, we give the evidence of the suitability of the DFT method through comparisons with high-level calculation methods. 

The CBS-Q/B3 composite calculation method uses this level for structure calculation, based on studies showing it results in improved accuracy [112], 

Chen and Bozzelli [86] have used the density functional calculations for oxygenated hydroperoxides and peroxy radical species they show that two different levels of B3LYP calculations, combined with use of isodesmic reaction analysis, result in good agreement with 

MP4 ab initio and CBSlq composite methods. B3LYP/6-311G(d.p) was also shown by Mebel et al. [33] to give similar enthalpies to values calculated by the G2M composite method, on a number of unsaturated peroxide peroxy species. 

B3LYP/6-311G(d,p) is chosen in the present work because it has a somewhat larger basis set than the commonly used B3LYP/6-31G(d,p) and it is still accurate for structure optimisation and force constant calculation on the molecules considered in this study. 

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