Nanoscale order in molecular systems from single crystal diffuse scattering

Diffuse scattering - the coherently scattered intensity that is not localised on the reciprocal lattice - contains a wealth of information about the local order (order on the nanoscale) in crystalline materials. Since molecules and atoms will respond most strongly to their local chemical environments, it is a valuable tool in understanding how structure leads to properties. However, at present its collection and analysis are relatively specialised. Monte Carlo (MC) computer simulation of a model structure has become a powerful and well-accepted technique for aiding the interpretation and analysis of diffuse scattering patterns. Its great strength is its flexibility - as long as an MC energy can be defined, a model can be developed and tested. At one extreme a very simplified model may be useful in demonstrating particular qualitative effects, while at the other a quantitative and very detailed description of disordered structures can be obtained. Examples discussed include new results concerning p-chloro-N-(p-chloro-benzylidene)aniline, a molecule showing various degrees of molecular flexibility.