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: pyro- and piezoelectric properties of BN nanotubes and optical signatures of organic molecules on Si(001) surface. BN nanotubes turn out to be excellent piezoelectrics, with response values significantly greater than those of piezoelectric polymers. However, their symmetry leads to exact cancellation of the total spontaneous polarization in ideal, isolated nanotubes. Breaking of this symmetry induces spontaneous polarization comparable to those of wurtzite semiconductors. Turning to organics on Si(100), we calculated the atomic structure and the optical signatures of a cyclopentene overlayer on Si(001). Cyclopentene can be used to attach a variety of organic molecules to Si devices, including DNA, and can therefore form a basis of a sensor structure. The spectra turn out to be highly structure-dependent and can therefore be used to monitor interface formation.