Multiscale Simulations of Quantum Structures

Recent advances in theoretical methods and parallel supercomputing allow for reliable ab initio simulations of the properties of complex materials. We describe two current applications: (i) electron transport properties of a Si/organic-molecule/Si junction and (ii) polar properties of polyvinylidene fluoride (PVDF) and its copolymers. For the molecular junction, our results provide a qualitative picture and quantitative understanding of the mechanism leading to negative differential resistance for a large class of small molecules. For ferroelectric polymers, the calculations show that their polarization described by cooperative, quantum-mechanical interactions between polymer chains, which cannot be viewed as a superposition of rigid dipoles. Nevertheless, the ab initio results lead to a simple parameterization of polarization in each monomer unit as a function of copolymer concentration.