The focus of this research is to study the quantum behavior of the repair process of cytosine and thymine pyrimidine dimer lesions, and of the TT(6,4) photoproduct. Molecular Dynamic and QM/MM modelling of a TT(6,4) lesion docked to TT(6,4) Photolyase [1] showed high mobility in the first twenty amino acid residues, no spontaneous proton donation between the protein and the lesion for chosen histidine protonation states, and suggests the residue HIE-379 as the most likely proton donor. A QM/MM study showed how using PM6 as the QM method can correct lesion bond lengths predicted during a molecular dynamics simulation with a GAFF2 force field used for the T=T dimer. Comparisons to results from the PARM99 force field [2] were made. A MM and a brief QM/MM study on the C=C dimer, showed that this lesion has a slightly shorter average C5-C5’ bond than the T=T dimer, but a very similar C6-C6’ bond length. The C5-C5’ bond length in the C=C dimer is less susceptible to breaking due to excess charge. With the specific localisation of charge, the C5-C5’ bond of the C=C dimer will break with electron addition. QM benchmark studies focused mainly on the C=C and C=T dimers, with some inclusion of the T=T and T=C dimers. Comparison with CC2 calculations showed that TD-DFT/CAM-B3LYP/aug-cc-pVTZ, was reliable for the calculation of the first excited triplet and doublet states of the C=C dimer. For the T=T dimer this method was shown to accurately calculate the T2, T5 , and T6 energy levels. CAM-B3LYP/aug-cc-pVTZ slightly overestimated the S1 excitation energy, for both the C=C and T=T dimer. For the T=T dimer, DFT/CAM-B3LYP/aug-cc-pVTZ gave the geometries closest to RI-CC2 calculations. The change in basis set and correlation exchange functional of the DFT method produced very different geometries for the anionic doublet and triplet states of the C=C, T=T, and C=T dimers, as did the inclusion or exclusion of the ‘PCM’ solvent. CAM-B3LYP was found to increase the thermal heat, electronic and zero point energy predicted, but reduce heat capacity and entropy, compared to B3LYP. In the gas-state, the singlet T=T dimer had a longer C5-C5’ bond than C6-C6’ bond, unlike the C=C and C=T dimers.