Results

The results of the series of CASTEP calculations may be seen in Table 3.3.

  
Table 3.3: Results of structure calculations.

Figures 3.5 through 3.7 show graphs of , and E respectively against supercell side length, and Figures 3.8 through 3.10 show these values plotted against cut off energy. These indicate that the calculations have converged with respect to cut off at a value of approximately 600 eV. The results have also converged for a supercell side length of approximately 5 Å. The maximum dimension of the molecule in a Cartesian direction is 1.4 Å suggesting a separation of at least 3.6 Å between ions of adjacent molecules.

  
Figure 3.5:

  
Figure 3.6:

  
Figure 3.7:

  
Figure 3.8:

  
Figure 3.9:

  
Figure 3.10:

The experimental value for the bond angle is , which agrees favourably with the value calculated by runs for which the cut off energy and supercell side length are well converged. In small molecules the N--H bond length varies between 1.01 Å and 1.04 Å, which comfortably contains the converged value calculated. The close agreement in results between the calculation run for 30 iterations and those run for 20 iterations indicates that the latter are converged within the necessary accuracy for this experiment.