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Conclusion

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.



Mr. Matthew D. Segall
Fri Jul 21 15:33:30 BST 1995