Applying periodic density functional theory methods for solid state chemical
physics – modelling inter and intra-molecular interactions.
The extension of quantum chemistry methods from small isolated molecules to periodic solids containing up to ~1000 atoms has allowed the focus of computational work in chemical physics to shift from intra-to inter-molecular interactions. In parallel, experimental methods have developed, notably in neutron scattering centres like the Institut Laue Langevin, which give better access in measurements to physical processes dominated by inter-molecular interactions. Furthermore the simple nature of the neutron-matter interaction leads to a direct calculation of the measured spectral profiles, hence facilitating the comparison between measurement and simulation. While high frequency dynamical modes are thought to play an important role in energy storage in biologically important reactions, it is the lower frequency modes, determined mainly by inter-molecular interactions, which are activated under ambient conditions.
The quantum tunnelling of molecular rotors is a simple, extremely sensitive probe of inter-molecular interactions and has been used to evaluate solid state density functional theory (DFT) methods in this context. Molecular vibrations constitute an intrinsically more complex problem, but the same DFT methods generally allow neutron scattering spectra to be accurately reproduced. In particular solid state methods are particularly well suited to modelling vibrational modes in hydrogen-bonded systems in which the inclusion of strong inter-molecular interactions can shift the single molecule modes by a factor of two. Results ranging from calculations on hydrogen-bonded molecular crystals (benzoic acid, urea, nucleosides) to recent, experimental and computational work on polymeric systems, like DNA and polypeptides (Kevlar, collagen), will be presented.