These experiments demonstrate that CASTEP is capable of performing
structural optimisation on individual molecules to a high degree of
accuracy. The analysis of Thymine indicates that the analysis of
biological molecules may be performed with confidence. Furthermore, the
use of a GGA exchange correlation functional appears to yield
greater accuracy than LDA. It must be remembered however that the
experimental structural data was obtained from a crystalline form of
thymine, while the calculations were performed for an approximately
free molecule. This could lead to differences in bond lengths, and it
is difficult to predict if this would affect the results positively or
negatively. The study of 
indicates that bond angles are
reproduced accurately by CASTEP in addition to bond lengths. This
further emphasises the confidence we may have in the structural results
obtained from CASTEP calculations.
The analysis of convergence indicates that a cut off energy of approximately 650 eV is necessary to ensure convergence in the total energy and structure of molecules containing species such as N, O and C found in biological molecules. Furthermore a minimum separation of approximately 3.5 Å is sufficient to ensure electronic isolation between adjacent molecules.
These calculations were the first results produced with the CASTEP
codes. Subsequent experience has indicated that some improvements
could be made to the methods used. In particular a larger number of
iterations would have been desirable to ensure a higher degree of
convergence in the 
studies and the convergence tests for
thymine. The results produced have however proved to be adequate for
their purpose and have provided encouragement regarding the accuracy
of the calculations.