Abstract
In this study, molecular cluster methods are used to simulate the crystalline environment of l-α-alanine. Two clusters are constructed based on neutron diffraction data, containing 7 and 14 molecules respectively. The geometry of a central alanine molecule is optimised at several levels of theory, while the surrounding molecules in the cluster are kept fixed in space. By evaluating the difference between the optimised and the experimental geometries, an assessment is made of the different levels of theory used. These vary from semi-empirical to full DFT-B3LYP treatments of the cluster. Special attention is paid to hybrid methods, involving the ONIOM scheme. These cluster methods are then used to study a radiation-induced radical of alanine, since they allow unbiased geometry optimisation of the radical under study. The influence of the theoretical treatment is considered on several calculated hyperfine coupling constants. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003