Results

A graph of the energy after each iteration for calculations of the static energy of the molecule may be seen in Figure 3.3. In addition Figure 3.4 shows a graph of the converged total energy against cut off energy.

  
Figure 3.2: The atomic structure of thymine.

  
Figure 3.3:

  
Figure 3.4:

The chemical structure of thymine is shown in Figure 3.2. The calculated equilibrium bond lengths for the LDA and GGA calculations and a comparison with the equivalent bond lengths obtained from crystallographic studies are shown in Tables 3.1 and 3.2 respectively. It should be noted that the reported lengths of the hydrogen bonds in the crystallographic structure are unusually short. This may be due to the effect of the crystallographic environment and hence typical molecular bond lengths for the C--H and N--H bonds are also noted for comparison.

  
Table 3.1: Calculated bond lengths in thymine using LDA energy functional.

  
Table 3.2: Calculated bond lengths in thymine using GGA energy functional.

The average magnitude of the percentage discrepancy in bond length between the LDA and crystallographic data, using typical molecular hydrogen bond lengths, is 1.26%. The same figure for the GGA calculation is 0.90%. In particular the discrepancy in the length of hydrogenic bonds is significantly greater for the LDA calculation than for the GGA result.